Reusable async client

[odelmarcelle]

Description

Clears out the session attribute of AsyncHttpConnection and AIOHttpConnection when calling close(), which allow re-using a previously closed AsyncOpensearch instance.

Issues Resolved

Closes #637

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[codecov[bot]]

Codecov Report

Attention: 5 lines in your changes are missing coverage. Please review.

Comparison is base (e323ab2) 72.12% compared to head (d4edaa1) 72.10%. Report is 1 commits behind head on main.

Files	Patch %	Lines
opensearchpy/connection/http_urllib3.py	60.00%	4 Missing :warning:
opensearchpy/connection/http_async.py	0.00%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #639 +/- ## ========================================== - Coverage 72.12% 72.10% -0.02% ========================================== Files 89 89 Lines 7939 7949 +10 ========================================== + Hits 5726 5732 +6 - Misses 2213 2217 +4 ```

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[odelmarcelle]

Turns out the issue affects clients using Urllib3HttpConnection which discard the pool, but RequestsHttpConnection works fine with my local testing.

I discovered this by adding test for the synchronous client, which apparently uses Urllib3HttpConnection. I'm not too sure where to put a test that would use RequestsHttpConnection.

[saimedhi]

Turns out the issue affects clients using Urllib3HttpConnection which discard the pool, but RequestsHttpConnection works fine with my local testing.

I discovered this by adding test for the synchronous client, which apparently uses Urllib3HttpConnection. I'm not too sure where to put a test that would use RequestsHttpConnection.

Hello @odelmarcelle , please put your test here for RequestsHttpConnection. And also DCO is failing. Please fix it. Thank you.

[odelmarcelle]

Resolved the DCO.

@saimedhi AFAIK, the test won't work there as there's not connection established to an opensearch instance. It should be somewhere under /test_server/, right? I didn't manage to work it out by myself.

One last comment though: Why are there two async connection classes defined? I'm not too sure to understand why AsyncHttpConnection and AIOHttpConnection co-exist. Especially as the former is the one recommended in the guides while most tests are based on the latter.

[dblock]

LMK if you prefer to merge as is, or if you think adding that assert is a good idea.