[feature] Implement /api/v1/conversations masto API endpoints for viewing direct message (dm) conversations

[tsmethurst]

We can handle direct messages sending/receiving just fine, but currently there's no easy way to view all dm conversations in progress. Mastodon uses /api/v1/conversations (and related endpoints) for this, so we should do the same so that clients (tusky, pinafore) can use the GtS API to view conversations.

Relevant links:

• https://docs.joinmastodon.org/methods/conversations/
• https://docs.joinmastodon.org/entities/Conversation/

(draft of) Required steps to make this work:

• Add Conversation gts model + frontend model
• Add 'conversations' package + logic in https://github.com/superseriousbusiness/gotosocial/tree/main/internal/api/client
• Add Conversation cache in internal/cache/gts.go
• Add Conversation db model + migration
• Add logic for storing / updating conversations when a new message in the conversation thread is received.
• Add logic for marking conversations as 'read'