Add Support for different api methods for asynchronous rest transport.

[ohmayr]

Add support for the following types of api methods in asynchronous rest transport:

• LROs
• Extended LROs
• Pagers
• Server Streaming

Follow up:

• As we add support for each of these api methods, we need to updated the is_rest_unsupported_method macro in test_macros.j2 file which guards the generation of tests for any of the above api methods.
• search for this issue number / url in the code and update the code wherever it's referenced.

Note: There are no plans of supporting client streaming api methods for sync or async rest transport.