[extension/bearerauthtoken] Custom auth header support for bearertokenauth

[omleteer]

Adding a config param for bearertokenauth to use a http header other than "Authorization" to read the token param from

Description

Today the bearertokenauth (BTA) by default fetches the auth token from the http "Authorization" header. By allowing multiple auth headers this enables easier attribution as well as decoupling of an extension from a pipeline.

Link to tracking issue

Fixes #36985

Testing

1. Default config creation ensures that Header value if not specified is set to "Authorization" (defaultHeader)
2. Ensured current unit tests work are working as intended.
3. Added extra test to validate behavior with this parameter set (and to ensure "defaultheader" fails)

Documentation

Please see the README.md update.