TCP Keep-Alive Environment Variable Support

[adev-code]

Currently, TCP Keep-Alive can be configured using Config object and ~/.aws/config file as per Configuration Doc. This PR adds environment variable support via BOTOCORE_TCP_KEEPALIVE.

Example Usage: export BOTOCORE_TCP_KEEPALIVE=True or true or os.environ['BOTOCORE_TCP_KEEPALIVE'] = 'True' or 'true'

Addresses: https://github.com/boto/botocore/pull/2268

[adev-code]

Tests and Configuration Documentation edit will be added.

[codecov-commenter]

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Codecov Report

:white_check_mark: All modified and coverable lines are covered by tests. :white_check_mark: Project coverage is 93.15%. Comparing base (8121342) to head (98e2bd8). :warning: Report is 41 commits behind head on develop. :exclamation: Your organization needs to install the Codecov GitHub app to enable full functionality.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #3579      +/-   ##
===========================================
- Coverage    93.17%   93.15%   -0.02%     
===========================================
  Files           68       68              
  Lines        15411    15411              
===========================================
- Hits         14359    14356       -3     
- Misses        1052     1055       +3     

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[adev-code]

Created a PR for Doc change to add BOTOCORE_TCP_KEEPALIVE

https://github.com/boto/boto3/pull/4642

[adev-code]

Changelog added