Make custom transport implementation possible without "requests" installed.

[lmazuel]

We will have situation in the future where "requests" will not be available at all, and people will write their own transport. Today, we expect "requests" to be available in a few places, making azure-core not truly independant.

See also:

• Investigate perf improvement by not importing "requests" when no HTTP use planned #11276
• [core] implement our own CaseInsensitiveDict #23347

[xiangyan99]

#25344