Dynamic jwks based on iss and aud

[FZambia]

Proposed changes

More generic version of #618

It's possible to extract vars from issuer and aud JWT claims using Go regexp named groups, then use these vars to construct JWKS endpoint dynamically. In this case JWKS endpoint may be set in config as template. Let's look how this may help in case of using different realms of Keycloak:

{
  "token_issuer_regex": "https://example.com/auth/realms/(?P<realm>[A-z]+)",
  "token_jwks_public_endpoint": "https://keycloak:443/{{realm}}/protocol/openid-connect/certs"
}

Two new options introduced:

• token_issuer_regex - match JWT issuer against this regex, extract named groups to variables for jwks endpoint construction.
• token_audience_regex - match JWT audience against this regex, extract named groups to variables for jwks endpoint construction.

TODO:

• when using token_issuer_regex and token_audience_regex make sure token_issuer and token_audience not used in the config - return an error on start in this case.

[FZambia]

@matpuk hello, probably this addresses your previous comment - maybe you'll be interested to check this out.