Create migration to point old destinations to ExternalStreamingDestination

Create a new column for ExternalStreamingDestination models that compose a unique key identifier that point to its old DestinationExternalAuditEventDestination` model

The column: legacy_destination_ref is the ExternalStreamingDestination ID column (PK)

When using the legacy_destination_ref, we should be able to use category to look up the appropriate old model and its data.

The new column id should validate on:

• uniqueness per category at the instance level
• uniqueness per category and group_id at the group level
• in both cases, allow nil