Replace nimbus ReadOnlyAuthorizationServerMetadata

[vafeini]

Currently VCI library is resolving (AuthorizationServerMetadataResolver) an OAuth server's metadata as CIAuthorizationServerMetadata, a typealias of nimbus' ReadOnlyAuthorizationServerMetadata.

CIAuthorizationServerMetadata is set as a property in CredentialOffer.

As ReadOnlyAuthorizationServerMetadata is not serializable this causes issues to clients of vci library when they need to serialize a resolved CredentialOffer.

• Implement CIAuthorizationServerMetadata as a data class instead of a typealias of ReadOnlyAuthorizationServerMetadata of nimbus. Its proprties should align with those exposed by ReadOnlyAuthorizationServerMetadata interface.