Add subscription for issuable weights

What does this MR do and why?

This MR add a GraphQL subscription for issuable weight updates and its trigger in the backend. Related to #362550 (closed) #371413 (closed).

How to set up and validate locally

⚠ Unfortunately, rmosolgo/graphiql-rails doesn't support Action Cable Subscriptions and there is no quick way to validate an instance of the added subscription. See https://github.com/rmosolgo/graphql-ruby/issues/1045#issuecomment-338306779 and https://github.com/rmosolgo/graphql-ruby/issues/1051.

subscription onIssuableWeightUpdated {
  issuableWeightUpdated(issuableId: "gid://gitlab/Issue/460") {
    ... on Issue {
      weight
    }
  }
}

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.