[ENHANCEMENT] Managed Saved Search for Single Features

[jcscottiii]

In order to support subscriptions for single Features, we should create system managed saved searches./

• You will likely want to create a new table using make spanner_new_migration and writing the definition for it.

These managed saved searches will be:

• Full managed for a feature during sync.

  • That means created when we see a new feature
  • Changed if we detect a feature id is moved.
  • Deleted when we detect a feature is deleted

• We also want to expose this new saved search ID in a field for a feature. That way the frontend can use that saved search ID to configure

• This will require modifying the query and the structs example to return it

• These types of saved searches will be of a different scope too