[Cosmos] work items for async preview

[simorenoh]

This issue will be used to track work items that should be completed while the async client is in preview mode. Any item with a checkmark will have its PR as a comment below as well.

• Move the asynchronous client away from using **kwargs to align with python3 best design practices
• Remove support for Python2 - RELEASED b2
• Add warning to sync client's populate_query_metrics flag if used in methods that don't need it and remove from documentation
• Deprecate read_offer in the sync api to align with read_throughput renaming in async client (Cosmos no longer references Offers in public docs as opposed to Throughput - may confuse users trying to use these functionalities)
  • what about this doc: https://docs.microsoft.com/en-us/rest/api/cosmos-db/offers ?
  • does this need to be updated to show the Python SDK's way of doing it as well? https://docs.microsoft.com/en-us/azure/cosmos-db/set-throughput
• Undocument/ remove documentation around build_options, cosmos_client_connection, connection_retry_policy for both clients - objects need to be private. For this one, add samples to show how a user might pass in retry/ policy config values as opposed to object instance
  • Will have to split this one into individual efforts for each since some cover a lot of area within the SDK
• Transition into using python3 typehints for async client
  • https://mypy.readthedocs.io/en/stable/cheat_sheet_py3.html - differences in python 3.9+ and python 3.5-, are we more inclined to 3.9 standards? - spoke with Travis, we're going with 3.5 since we support 3.7+ starting 2022
• GetAuthorizationHeader documentation should be removed from sync client and then made private, also bring in method with proper naming to not create breaking changes.
  • This method is being used both in tests and in samples for non-partitioned container operations using rest API, in order to remove usages of the method these tests/samples would have to be removed too
• Verify and split proof GONE status code for partition key refresh - RELEASED SYNC b2, ASYNC b3
• Edit async client user agent to be different for ease of use when debugging - RELEASED b2
• before we go GA we should make most of these files in aio directory private
• [Cosmos] move is_system_key logic to be computed only once on container usage #23169 - RELEASED b4

[simorenoh]

Split proof sync client queries: #22237 - RELEASED IN b2

[simorenoh]

Split proof async client queries: #22261 - RELEASED IN b3

[simorenoh]

Deprecate read_offer in clients to align with read_throughput renaming: #22262

[simorenoh]

Edit async client user agent to be different for ease of use when debugging: #22263 - RELEASED IN b2

[simorenoh]

Add warning to sync client's populate_query_metrics flag if used in methods that don't need it and remove from documentation: #22264

[simorenoh]

Transition into using python3 type hints for async client: #23933
also includes changes for moving the asynchronous client away from using **kwargs to align with python3 best design practices

[simorenoh]

GetAuthorizationMethod documentation should be removed and then made private with proper method naming: #22284

[simorenoh]

Remove support for Python2: #22475 - RELEASED IN b2

[simorenoh]

Move is_system_key logic: #23821 - RELEASED IN b4

[simorenoh]

Undocument/ remove documentation around build_options, cosmos_client_connection, connection_retry_policy for both clients: #23945
All of the methods and classes mentioned are already private, live in a private class, or are marked for deprecation with the PR above.