feat: persist and clean upgrade intent msg, ratelimit, and API

[hopeyen]

Description

• Persist a HashMap of upgrade intent messages - require unique subgraph id
• When operator receives an ui msg, check the hashmap for the subgraph id
  • if there's no existing record, add msg to the hashmap and offchain sync
  • if there's existing record, check for the existing nonce. Do nothing if there has been a recent upgrade, otherwise replace the old message and offchain sync
• Added Http service with optional subgraph_id filter
• No explicit cleaning as only 1 message per topic is permitted

Issue link (if applicable)

Resolves #25
Resolves #40

Checklist

• Are tests up-to-date with the new changes?
• Are docs up-to-date with the new changes? (Open PR on docs repo if necessary)