Observers missing API for triggers targetting components on a given Entity

[james-j-obrien]

What problem does this solve or what need does it fill?

• Only unsafe APIs currently exist that allow you to specify both target components and entities, used internally for OnAdd, OnInsert, etc. This behaviour should be accessible to users via the safe APIs

What solution would you like?

• Implement TriggerTargets for a combination of Entity and ComponentId.

Additional context

This API seems to have been dropped in the refactor from builder to TriggerTargets, we should probably try and check in a fix before release although it is possible to just manually implement TriggerTargets.