Metadata for Multiple Trust-Frameworks

There are situations, where multiple trust-frameworks need to be supported by one issuer for the same credentials. Especially for credentials that are issued in (trust-framework) border-regions (e.g. Switzerland vs EU). As such the standard would benefit from a explicit specification on how to do this.

@c2bo suggested to use application/jose+json for such purposes as it is used in OpenID4VP. This fits the recently discussed and merged change to remove signed_metadata (c.f. https://github.com/openid/OpenID4VCI/pull/520) in favour of an Accept header to request a specific Content-Type (or format) for the issuer metadata.

As such we should extend https://github.com/openid/OpenID4VCI/blob/60a33f06a3f3073c902239ddf792e028fa890685/openid-4-verifiable-credential-issuance-1_0.md?plain=1#L1261-L1264 to include

* a JWS json containing the Credential Issuer Metadata in its payload, offering multiple different signatures, using the media type `application/jose+json`.  This serves use cases where the Issuer requests Credentials belonging to different trust frameworks and, therefore, needs to authenticate in the context of those trust frameworks

(See for the previous discussion on signed_metadata: https://github.com/openid/OpenID4VCI/issues/528 )

Open openid-4-verifiable-credential-issuance-1_0.md at lines 1261-1264 and review the discussion in issue 528 and pull request 520 for context. Confirm the proposed application/jose+json wording fits the metadata content-type description; done means the specification explicitly describes a JWS JSON payload supporting multiple signatures and trust frameworks.