Allow client to authenticate using private_key_jwt when using OIDC node

When using OIDC authentication node, the client always authenticates to OIDC provider provider using client_secret_post.

Exit

Node could be configured so that the client uses private_key_jwt for authentication if required.
The implementation should also define a URL to expose the client's public key.

Acceptance Criteria

• Social ID provider node can be configured to use private_key_jwt instead of client_secret_post
• the request contains the appropriate additional request parameters for using private_key_jwt
• the jwt's sub is AM's client ID
• an endpoint exists where OPs can retrieve the public key to verify the JWT signature
• the JWT is signed-then-encrypted