Support for sdk_ua_app_id config setting

[jonemo]

This PR adds support for the AWS_SDK_UA_APP_ID environment variable and sdk_ua_app_id setting in the shared configuration file. This is already supported by some other AWS SDKs today and will in future be the recommended way for third party apps that use botocore to inject an identifier into the User-Agent header. Botocore will also continue supporting the user_agent and user_agent_extra config settings for backwards compatibility.

For test coverage, this PR introduces new functional testing for all user agent header related config settings. While there are existing tests for user agent settings in unit/test_client.py, unit/test_session.py, and unit/test_session_legacy.py, they either test session-level settings (which get overridden or modified by client-level config) or mock the legacy endpoint resolver.

Open questions where for which I'd be particularly interested to hear feedback:

1. Naming of the config variable. Right now I made the config variable name similar to existing config variables but different from the config file setting name.
2. Test fixtures: I wonder if there is an easier way to stub the requests that requires less setup and simply captures all outgoing requests. Probably not because it would require somehow aborting the request before the response is parsed...

[nateprewitt]

Where are we enforcing this currently?

[codecov-commenter]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (8bd1a4d) 93.29% compared to head (f3d1452) 93.30%.

❗ Current head f3d1452 differs from pull request most recent head e3efab1. Consider uploading reports for the commit e3efab1 to get more accurate results

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Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #2942   +/-   ##
========================================
  Coverage    93.29%   93.30%           
========================================
  Files           64       65    +1     
  Lines        13588    13600   +12     
========================================
+ Hits         12677    12689   +12     
  Misses         911      911           

Impacted Files	Coverage Δ
botocore/config.py	100.00% <ø> (ø)
botocore/configprovider.py	94.09% <ø> (ø)
botocore/args.py	100.00% <100.00%> (ø)
botocore/useragent.py	100.00% <100.00%> (ø)

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[jonemo]

Closing this in favor of a larger PR with more user agent header changes. Just too many interconnected pieces to cleanly separate this into smaller pieces...