[Federation] aud claim should be removed from Entity Statement

[Giuseppe De Marco]

the fetch endpoint doesn't have any client authentication mechanisms or nothing valuable to known who's the audience of the issued statement

For this reason the aud claim in the entity statement should be removed

[Giuseppe De Marco]

The PR that closes this issue is https://bitbucket.org/openid/connect/pull-requests/530

[Vladimir Dzhuvinov]

The “aud” claim is used in explicit registration requests and responses:

https://openid.net/specs/openid-connect-federation-1_0.html#appendix-A.3.2

(not in response to an authenticated request, but for a “pushed” EC or ES that was minted for the OP, resp RP)

[Giuseppe De Marco]

Ok @Vladimir Dzhuvinov I propose to keep this PR open by changing its scopes including in the next commits all the required changes to improve the text related to the explicit registration

[Giuseppe De Marco]

not relevant