PkceCodeChallenge::new wrap an existing challenge

[Timshel]

Add the ability to wrap an existing challenge.

I realize it was probably a conscious choice not to implement it as mentioned in this other comment:

This type intentionally does not implement Clone in order to make it difficult to reuse PKCE challenges

But at the same time I'm using server side OpenIDConnect authorization code flow with client_id and client_secret and I would like to be able to use pkce too and pass the challenge created by the client.

Edit: Realized afterward that I could just use add_extra_param but I think it would be cleaner to allow to build the PkceCodeChallenge.