feat: allow to set a signature host for tunnel usage with async client

[brunomurino]

Description

When creating an Async client, you can now pass a custom 'host' header that will be used for signing the AWS request, if using AWS authentication. This allows accessing OpenSearch via SSH/SSM tunnel, since when doing so the local port of OS would be, most of the time, 443 which is protected.

Issues Resolved

Closes #184

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.
For more information on following Developer Certificate of Origin and signing off your commits, please check here.

[codecov]

Codecov Report

Attention: Patch coverage is 91.66667% with 1 line in your changes missing coverage. Please review.

Project coverage is 70.44%. Comparing base (ba715b9) to head (a6324e8).
Report is 75 commits behind head on main.

Files with missing lines	Patch %	Lines
opensearchpy/helpers/asyncsigner.py	91.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #862      +/-   ##
==========================================
- Coverage   71.95%   70.44%   -1.51%     
==========================================
  Files          91      125      +34     
  Lines        8001     9311    +1310     
==========================================
+ Hits         5757     6559     +802     
- Misses       2244     2752     +508     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.