Mex 706 token subsctiptions redis key watcher

[EmanuelMiron]

Reasoning

• Token Metadata Subscription

Proposed Changes

• Create a new subscription endpoint for Token Metadata which will be publish data on Redis key change
• Whenever token.tokenMedata.* keys from Redis will be changed, new data will be published

How to test

subscription {
  tokenMetadataChanged {
    identifier,
    price
  }
}