Creating metadata object to not expose raw protobuf

[jevy]

The optionalTransactionMetadata is using the Struct object from the google protobuf package. Instead of exposing that to the client, it would be better to encapsulate that within the authzed library. This PR creates a utility to build the struct object.

So this enables the client to add relationships this way:

const writeRelationshipRequest = v1.WriteRelationshipsRequest.create({
  updates: [relationshipUpdate],
  optionalTransactionMetadata: v1.createStructFromObject({
    organization: licenseId,
  }),
});