Add new flags "http-stack" and "async-http-stack"

[lmazuel]

corehttp will not install any HTTP stack by default, and will require one installed by the SDK itself.

This issue is to add a flag that with those values:

• http-stack:
  • requests
  • httpx
• async-http-stack:
  • aiohttp
  • httpx

Default behavior if no flag is requests and aiohttp

Once this flag is set, we need to generate different code:

• We provide extra_requires to make this experience easier. Change setup.py accordingly to corehttp[requests] or corehttp[httpx] or the combined version corehttp[requests, httpx]
• Create the right transport in client constructor code to create a transport of the right type, and pass it to the PipelineClient (resp. AsyncPipelineClient. Beware to pop customer 'transport' kwarg as well. Also, the import needs to be inside the constructor, the import is not done if the transport kwarg is set. For instance:

# Pseudo-code, 
transport = kwargs.pop("transport", None)
if not transport:
   from corehttp.transport.httpx import HttpxTransport
   transport = HttpxTransport(**kwargs)