Dictionary property for non-standard extensions

The RegistrationRequest class could have a IDictionary property for adding key-values to be sent along with the request for non-standard extensions.

Example:

var request = new RegistrationRequest
{
    ClientName = "example",
    // ...
};
request.Extensions.Add("scope", "openid profile billing");

[leastprivilege]

I updated the docs. Feel free to do a PR if more information is needed

https://identitymodel.readthedocs.io/en/latest/client/dynamic_registration.html

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