Implement token introspection helper

[emgrav]

Steps to implement

• Implement helper function to perform token introspection against Zitadel

Input

• Zitadel access token

Output

TBD, depends on https://github.com/famedly/vzd-fhir-directory-proxy/issues/88

https://zitadel.com/docs/guides/integrate/token-introspection/private-key-jwt https://zitadel.com/docs/guides/integrate/service-users/private-key-jwt

[mzaniolo]

On vzd-fhir-directory-proxy we decided to don't do introspection but rather use a JWT and validate the signature. We won't need the introspection helper anymore but it would be good to have a helper to get the public keys from Zitadel. We could even have the helper to return only the key we are interested in.

Zitadel docs