Allow building a ClientRegistration from provided configuration

[heruan]

This adds the fromOidcConfiguration method to ClientRegistrations to build a ClientRegistration from a map representation of an OpenID Provider Configuration Response.

This is useful when the OpenID Provider Configuration is not available at a well-known location, or if custom validation is needed for the issuer location (e.g. if the issuer is only reachable via a back-channel URI that is different from the issuer value in the configuration).

Example usage:

RequestEntity<Void> request = RequestEntity.get(metadataEndpoint).build();
ParameterizedTypeReference<Map<String, Object>> typeReference = new ParameterizedTypeReference<>() {};
Map<String, Object> configuration = rest.exchange(request, typeReference).getBody();
// Validate configuration.get("issuer") as per in the OIDC specification
ClientRegistration registration = ClientRegistrations.fromOidcConfiguration(configuration)
    .clientId("client-id")
    .clientSecret("client-secret")
    .build();

Fixes: gh-14633

[rwinch]

Thanks for the Pull Request! This is now merged into main 😄