Add support for signed_metadata in Credential Issuer Metadata

[dzarras]

Quoting the spec:

signed_metadata:

String that is a signed JWT. This JWT contains Credential Issuer metadata parameters as claims. The signed metadata MUST be secured using JSON Web Signature (JWS) [RFC7515] and MUST contain an iat (Issued At) claim, an iss (Issuer) claim denoting the party attesting to the claims in the signed metadata, and sub (Subject) claim matching the Credential Issuer identifier. If the Wallet supports signed metadata, metadata values conveyed in the signed JWT MUST take precedence over the corresponding values conveyed using plain JSON elements. If the Credential Issuer wants to enforce use of signed metadata, it omits the respective metadata parameters from the unsigned part of the Credential Issuer metadata. A signed_metadata metadata value MUST NOT appear as a claim in the JWT. The Wallet MUST establish trust in the signer of the metadata, and obtain the keys to validate the signature before processing the metadata. The concrete mechanism how to do that is out of scope of this specification and MAY be defined in the profiles of this specification.

• Add a way to produce issuer-signed metadata (i.e. sub is iss)
• Add configuration options to control the signing key used for metadata