RDS ReadReplicaDBInstanceIdentifiers and SourceDBInstanceIdentifier show db-name for cross-region replication

[Tmilly]

Currently in all cases, moto will use a database's name for the ReadReplicaDBInstanceIdentifiers and SourceDBInstanceIdentifier fields in the RDSBackend.

This aligns with AWS's states for same-region replication. However for cross-region replication, AWS provides full arns for ReadReplicaDBInstanceIdentifiers and SourceDBInstanceIdentifier. Moto does not reflect this behavior for cross-region replication.

Can we update this for cross-region support to ensure moto stores/uses the full arn for its DBInstanceIdentifiers?

Reproducible steps:

1. Create a same-region replication (primary and replica in the same region) in AWS, and run a describe call on both the primary and replica. Notice both SourceDBInstanceIdentifier and ReadReplicaDBInstanceIdentifiers align with just the database name
2. Create a cross-region replication (primary in one region, replica in a second region) in AWS, and run a describe call on both the primary and replica. Notice both SourceDBInstanceIdentifier and ReadReplicaDBInstanceIdentifiers align with just the resource's full arn.
3. Create a cross-region replication using moto, and run a describe call on both the primary and replica. Notice both SourceDBInstanceIdentifier and ReadReplicaDBInstanceIdentifiers come back as the database name. (This does not align with AWS's behavior)

[bblommers]

Thanks for raising this @Tmilly, and welcome to Moto - marking this as a bug.

PR's are always welcome, if you'd like to tackle this!