Add endpoint for getting posts from multiple communities

[Ategon]

Requirements

• Is this a feature request? For questions or discussions use https://programming.dev/c/pangora
• Did you check to see if this issue already exists?
• Is this only a feature request? Do not put multiple feature requests in one issue.
• Is this a backend issue? Use the lemmy-ui repo for UI / frontend issues.

Is your proposal related to a problem?

Currently when doing multi community functionality it doesnt give posts in the correct order since they get queried separately and then I cant access how active sort sorted them for example

Describe the solution you'd like.

Add a new argument to GetPosts called community_ids that when populated it pulls from instead of community_id (or make community_id accept both). (same for community_name)

Describe alternatives you've considered.

Doing sorting in the frontend - not feasible with all of the different sorts, lack of data, and slowness

Additional context

Also let it enter ids or names that dont actually exist and just ignore those. Also allow wildcards to allow communities with a name name on any instance (and ignore ones that dont work)