add small note about client id parameters for dc api

[TimoGlastra]

Fixes #436

[TimoGlastra]

Some notes I also shared with @peppelinux over email:

After re-reading the section on DC API, I must say that there is already a line indicating the set of parameters can be extended: "Additional request parameters MAY be defined and used with OpenID4VP over the DC API.".

So it may not be needed to merge the clarification, although me and another engineer on our team both were in doubt about the restrictions from the supported parameters.

[jogu]

Some notes I also shared with @peppelinux over email:

After re-reading the section on DC API, I must say that there is already a line indicating the set of parameters can be extended: "Additional request parameters MAY be defined and used with OpenID4VP over the DC API.".
So it may not be needed to merge the clarification, although me and another engineer on our team both were in doubt about the restrictions from the supported parameters.

The list of parameters that can be used in the DC API defined in section A.2 is explicitly prefixed with:

Out of the Authorization Request parameters defined in [@!RFC6749] and (#vp_token_request), the following are supported with OpenID4VP over the W3C Digital Credentials API

I'm not actually sure how to interpret that text, given that trust_chain is defined in OID Federation.

The intent of all this text was to make sure that not all the request parameters from https://www.iana.org/assignments/oauth-parameters/oauth-parameters.xhtml#parameters automatically became allowable to use in the DC API. For example, state and code_challenge are deliberately excluded from the list as they serves no purpose in the DC API.

So likely your proposed text makes sense as I can't currently think of a case where a client id scheme might define a parameter that doesn't make sense in the DC API.

[jogu]

I think it actually makes sense to add this note further up - either before line 2176 or before 2180? And word it more like:

Parameters defined by a specific client identifier scheme (such as the trust_chain parameter for the OpenID Federation client id scheme) are also supported over the W3C Digital Credentials API.