Allow for optional introspection URL

[weichweich]

Allows to use the Client with an optional IntrospectionURL.

[silwol]

Any chance to get this merged, @ramosbugs ? I'd love to upgrade from openidconnect v3 to v4, this change would be the enabler for that.

[aitiotekt]

Perhaps it would be best to add all of them (excluding set_auth_uri_option, which is required in the openidconnect scenario):

• set_introspection_url_option
• set_revocation_url_option
• set_device_authorization_url_option
• set_token_uri_option
• set_user_info_url_option

Since the oauth2 crate supports the first four, and it would also be best to add the last one for symmetry.

CAN FIND THEM HERE