extensions: make jwt_authn filter token-parsing more flexible (#6355)

[larrywest]

Description: modifies jwt_authn filter's ExtractorImpl extract method to use the from_headers's value_prefix tag more precisely, allowing syntax like "tag=<JWT>,other=xxx" rather than simply taking the remainder of the string as the JWT candidate.

See Issue #6355 for full description. Should be backwards-compatible with existing uses of jwt_authn.

Risk Level: Medium (scope: affects JWT authentication)

Testing: Unit testing was added to test/.../jwt_authn/extractor_test.cc. Since Extractor itself does not validate the JWT, only the parsing is tested.

Docs Changes: see "Further header options" section added to api/envoy/config/filter/http/jwt_authn/v2alpha/README.md

Release Notes:

Adds enhancement per Issue #6355, so that deployments can use a wider variety of HTTP header syntaxes to pass JWTs and have them authenticated by the jwt_authn filter. Backwards-compatible with existing usage.

JWT authentication with the jwt_authn HTTP filter now permits header syntax like the following:

Authorization: Bespoke jwt=eyJhbGciOiJS...ZFnFIw,extra=7,realm=123