Add SOCKS proxy support

[dbevacqua]

Although this "works for me", it's still work in progress and needs at least some more tests in test_http_session.py to cover cases where URLLib3Session is configured with SOCKS proxies. Before going any further I wanted to verify the general approach.

One particular question is how to avoid warnings from importing urlib3.contrib.socks when socks (an optional dependency) is not available.

[codecov-commenter]

Codecov Report

Merging #2081 into develop will decrease coverage by 0.08%. The diff coverage is 73.91%.

@@             Coverage Diff             @@
##           develop    #2081      +/-   ##
===========================================
- Coverage    93.11%   93.03%   -0.09%     
===========================================
  Files           60       60              
  Lines        11096    11136      +40     
===========================================
+ Hits         10332    10360      +28     
- Misses         764      776      +12     

Impacted Files	Coverage Δ
botocore/awsrequest.py	95.31% <36.36%> (-2.26%)	:arrow_down:
botocore/httpsession.py	91.08% <85.71%> (-1.40%)	:arrow_down:

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[hnousiainen]

I have a need to access AWS endpoints over SOCKS5 proxy and I've tested this modification successfully in that environment. Are you still working on this PR, and is there anything I could help with?

[zhan9san]

@dbevacqua

Are you still working on this feature? Would you mind if I pick it up?

[dbevacqua]

@zhan9san please be my guest

[tim-finnigan]

Thank you for creating this PR. After bringing this up for discussion with the team we decided to continue tracking this request in https://github.com/boto/botocore/pull/2541 going forward, with the latest update here: https://github.com/boto/botocore/pull/2541#issuecomment-1666001438