expose http_session_cls in AioConfig

[jakkdl]

Description of Change

This is split out from #1085, partly to make the diff smaller, and partly because it could on its own open up another avenue of achieving the same goal - where an end user/plugin/etc could supply a custom class that e.g. doesn't depend on aiohttp.

If #1085 is considered too crude/messy, and/or not wanting the tough tradeoff between maintaining both aiohttp+httpx vs dropping aiohttp-only functionality, this + a separate PR to replace asyncio usage with anyio could perhaps achieve #749 while still only using aiohttp in this repository (for now).

Assumptions

Replace this text with any assumptions made (if any)

Checklist for All Submissions

• I have added change info to CHANGES.rst
• If this is resolving an issue (needed so future developers can determine if change is still necessary and under what conditions) (can be provided via link to issue with these details): groundwork for support pluggable http clients/coroutine frameworks (anyio/httpx) #749
  • Detailed description of issue
  • Alternative methods considered (if any)
  • How issue is being resolved
  • How issue can be reproduced
• If this is providing a new feature (can be provided via link to issue with these details):
  • Detailed description of new feature
  • Why needed
  • Alternatives methods considered (if any)

Checklist when updating botocore and/or aiohttp versions

• I have read and followed CONTRIBUTING.rst
• I have updated test_patches.py where/if appropriate (also check if no changes necessary)
• I have ensured that the awscli/boto3 versions match the updated botocore version

[thehesiod]

go ahead and set the next patch version with date and I'll merge and release

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 86.29%. Comparing base (8294d52) to head (d12348c).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1102      +/-   ##
==========================================
+ Coverage   86.25%   86.29%   +0.04%     
==========================================
  Files          62       62              
  Lines        5891     5910      +19     
==========================================
+ Hits         5081     5100      +19     
  Misses        810      810              

Flag	Coverage Δ
unittests	86.29% <100.00%> (+0.04%)	⬆️

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[jakkdl]

go ahead and set the next patch version with date and I'll merge and release

done!