Adding support for hashed client_secrets

[markmeeus]

We are currently migrating from an older oauth implementation to node-oidc-provider.
We have our current clients stored in our backend, but not their client_secrets. In our current implementation we validate the secret against a hashed secret we have in our db.

It looks like it is not supported, but I'm not sure it's by design or not.

I think it could interesting to add an extension point to the Client class where users can implement the hashing function on the client_secret.

   compareClientSecret(actual) {
      actual = instance(provider).configuration.hashClientSecret?.(this, actual) || actual
      return constantEquals(this.clientSecret, actual, 1000);    
    }

If this sounds viable, I'd be happy to provide a PR for it

[panva]

It looks like it is not supported, but I'm not sure it's by design or not.

The compareClientSecret prototype method on a Client already exists and you can overload it. There's no need for more.