Spring Security Add repository for returing Asserting Party Metadata

Like RelyingPartyRegistrationRepository, it would be nice to have an interface to represent loading asserting party metadata. This would make working with OpenSAML's MetadataResolver considerably easier.

A contract like the following may suffice:

public interface AssertingPartyMetadataRepository extends Iterable<AssertingPartyMetadata> {
    default AssertingPartyMetadata findByEntityId(String entityId);

    Iterator<AssertingPartyMetadata> iterator();
}

Note that AssertingPartyMetadata is a proposed interface to allow for easy adaptation from DB or other third-party representations.

A handy first implementation of this interface would be one that worked with an OpenSAML MetadataResolver.