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

• [x] I have added change info to CHANGES.rst
• [x] 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 #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[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

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

Additional details and impacted files

```diff @@ 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](https://app.codecov.io/gh/aio-libs/aiobotocore/pull/1102/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=aio-libs) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/aio-libs/aiobotocore/pull/1102/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=aio-libs) | `86.29% <100.00%> (+0.04%)` | :arrow_up: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=aio-libs#carryforward-flags-in-the-pull-request-comment) to find out more.

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.

[jakkdl]

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

done!