Protect against malicious queries

The Scala implementation of GraphQL (Sangria) provides a way for being protected against malicious queries.

More info: http://sangria-graphql.org/learn/#protection-against-malicious-queries

Could be very useful if the executor could support the following:
- Maximum query complexity based on analysis of the query
- Limit query depth

Start by reviewing the executor and the linked Sangria documentation on protection against malicious queries. Define how query-complexity analysis and maximum depth should be exposed and validated, then identify the relevant tests or entry points before implementation. Done means both limits are supported and their behavior is covered by tests.

索引モデルが issue の本文から書いたものです。