feature: persistent read-only event subscription functionality

[ra0x3]

Background

• We have had several beta testers/users specifically request websocket functionality
• Specifically these users want to push events from their indices downstream to their owned and operated sinks
• Brandon Kite previously mentioned we could provide this via GraphQL Subscription
  • This is an interesting idea, but requires integration with a database
  • Specifically multiple users have asked for websocket functionality that is not connected to a database
    • These users are essentially using the indexer as a ETL middle-man/interface to the Fuel VM

Implementation

• For the time being we will just focus on websockets for WASM execution (it will be pretty straightforward to roll your own WS in native execution)
• The WS might have to be coupled with the database -- but there would need to be an option to not touch the database as well
  • I.e., allow users to save_and_push their entities (i.e., save to the DB then push downstream via WS)
  • But also allow users to just push their entities (i.e., push their entities downstream via WS without involving a DB)

[ra0x3]

This issue primarily mentions "websockets" but the implementation does not have to be that exactly.

[ra0x3]

Blocked by https://github.com/FuelLabs/fuel-indexer/issues/1145