Move AsyncOperation and AsyncBlockOperation to WordPressShared

[mokagio]

I run into those as dependencies for the Core Data logic, see #24166 and #24165. There's lots to move in that PR, so I pushed this little self-contained change in the meantime.

I got bitten a few time by these changes where the app seems to compile fine but then crashes at runtime. So, I tested this "properly".

[wpmobilebot]

📲 You can test the changes from this Pull Request in WordPress Alpha by scanning the QR code below to install the corresponding build.

	App Name	WordPress Alpha
	Configuration	Release-Alpha
	Build Number	pr24185-0dba621
	Version	25.8
	Bundle ID	org.wordpress.alpha
	Commit	0dba621
	App Center Build	WPiOS - One-Offs #11626

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[wpmobilebot]

📲 You can test the changes from this Pull Request in Jetpack Alpha by scanning the QR code below to install the corresponding build.

	App Name	Jetpack Alpha
	Configuration	Release-Alpha
	Build Number	pr24185-0dba621
	Version	25.8
	Bundle ID	com.jetpack.alpha
	Commit	0dba621
	App Center Build	jetpack-installable-builds #10657

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