DPoP add-on has not been implemented properly

[csc-jm]

We utilize the RPHandler from this library for a project and recently needed to add DPoP mechanism to the authorization flow. I attempted to configure it as shown here: https://github.com/IdentityPython/idpy-oidc/blob/main/demo/oauth2_add_on_dpop.py

However, the OIDC client with DPoP enabled wasn't receiving the DPoP nonce nor header as described in RFC 9449 from the RPHandler.finalize() call. After some investigation, I came to the conclusion that the DPoP add-on from this library might be lacking functionality a bit.