Add support for the _filter search parameter in Subscription.criteria

[ThatOneBro]

Currently it's not possible to use _filter search parameter for Subscription.criteria. This is because the internal function matchesCriteria() makes use of the in-memory search, which currently has some limitations, one of which is the inability to use _filter.

_filter can be really useful, especially for complex queries where a logical OR is needed, so it would be great if we could get this added to Subscription.criteria, especially once WebSocket subscriptions are available for usage.

For example, a criteria for Communication resources representing a live chat session could be written as such:
Communication?patient={patientId}&_filter=(sender re Practitioner/123 or recipient re Practitioner/123)

However in the current system without _filter, we would have to create two Subscriptions, each with a criteria representing either side of the or operator in the _filter parameter above. That means adding the _filter search parameter for Subscription.criteria could reduce total subscriptions across all clients by half for a chat service modeled by Medplum's WebSocket subscriptions.

I can see two solutions; either seems like a decent path forward for.

1. Add the appropriate _filter query parameter to the in-memory search implementation. Probably better for performance.
2. Migrate subscription matchesCriteria() to database search. Maybe less work (?)

[Dharshan-K]

Hi, @ThatOneBro , I am interested in this issue. Can you guide to implement this feature.