graphql: added caching for validation phase

[dotansimha]

Closes #3077

TODO

• Add global cache for validation results
• Make sure cache does not cause false positives due to hash calculation

Overview of this PR

Since validate is static (give a query and a schema it will always produce the same result), we can cache it to avoid running it again and again if a given query hash is the same.

Benchmarks

Measured on M1 machine, in --release mode.

Phase	Without Cache	With HashMap
validate	~1.44ms	<=1µs

GraphQL query used for testing

{
  purposes(first: 100, subgraphError: allow) {
    id
    sender {
      id
      ...SenderFields
    }
    ...PurposeFields
  }
}

fragment PurposeFields on Purpose {
  purpose
  createdAt
  transactionHash
}

fragment SenderFields on Sender {
  address
  createdAt
  purposeCount
}

It seems like parse takes ~60µs so we don't have to do any significant optimizations at the moment.

[dotansimha]

After a discussion with @lutter , we can drop caching validations at the moment, since it's not adding a real overhead (~1ms).