[d16-4] [linker] Ensure we can remove NSUrlSessionHandler if unused

[monojenkins]

Moving NSUrlSessionHandler into the platform assemblies (e.g.
Xamarin.iOS.dll) instead of System.Net.Http.dll was not optimal as it
prevented the linker to remove it when the application did not use it.

This shows a lot in an "helloworld" type of application (see below)
but in real life most of the removed code gets used by something else
(and is included.

Directories / Files                                                              helloworld-d16-4.app helloworld-fixed.app         diff            %
./
        AppIcon60x60@2x.png                                                             2,632        2,632            0       0.00 %
        AppIcon76x76@2x~ipad.png                                                        3,125        3,125            0       0.00 %
        archived-expanded-entitlements.xcent                                              181          181            0       0.00 %
        Assets.car                                                                     75,688       75,688            0       0.00 %
        embedded.mobileprovision                                                        8,456        8,456            0       0.00 %
        helloworld                                                                  4,700,256    3,915,936     -784,320     -16.69 %
        helloworld.aotdata.arm64                                                        1,448        1,432          -16      -1.10 %
        helloworld.exe                                                                  6,144        6,144            0       0.00 %
        Info.plist                                                                      1,712        1,712            0       0.00 %
        mscorlib.aotdata.arm64                                                        406,080      364,104      -41,976     -10.34 %
        mscorlib.dll                                                                  561,664      501,760      -59,904     -10.67 %
        NOTICE                                                                            159          159            0       0.00 %
        PkgInfo                                                                             8            8            0       0.00 %
        System.aotdata.arm64                                                           17,008          936      -16,072     -94.50 %
        System.Core.aotdata.arm64                                                       2,432            0       -2,432    -100.00 %
        System.Core.dll                                                                 4,608            0       -4,608    -100.00 %
        System.dll                                                                     32,768        5,120      -27,648     -84.38 %
        System.Net.Http.aotdata.arm64                                                  31,648            0      -31,648    -100.00 %
        System.Net.Http.dll                                                            29,184            0      -29,184    -100.00 %
        Xamarin.iOS.aotdata.arm64                                                      62,544       33,464      -29,080     -46.50 %
        Xamarin.iOS.dll                                                                92,672       53,248      -39,424     -42.54 %
./_CodeSignature
        CodeResources                                                                   7,575        6,655         -920     -12.15 %
./LaunchScreen.storyboardc
        01J-lp-oVM-view-Ze5-6b-2t3.nib                                                  1,831        1,835            4       0.22 %
        Info.plist                                                                        258          258            0       0.00 %
        UIViewController-01J-lp-oVM.nib                                                   896          896            0       0.00 %
./Main.storyboardc
        BYZ-38-t0r-view-8bC-Xf-vdC.nib                                                  1,836        1,832           -4      -0.22 %
        Info.plist                                                                        258          258            0       0.00 %
        UIViewController-BYZ-38-t0r.nib                                                   916          916            0       0.00 %

Statistics

Native subtotal                                                                     4,700,256    3,915,936     -784,320     -16.69 %
    Executable                                                                      4,700,256    3,915,936     -784,320     -16.69 %
    AOT data *.aotdata                                                                      0            0            0          -

Managed *.dll/exe                                                                     727,040      566,272     -160,768     -22.11 %

TOTAL                                                                               6,053,987    4,986,755   -1,067,232     -17.63 %

Backport of #7151.

/cc @spouliot

[chamons]

Approved for P3.

[monojenkins]

Build success
✅ Build succeeded
✅ API Diff (from stable)
✅ API Diff (from PR only) (no change)
✅ Generator Diff (no change)
✅ Test run succeeded