Support of a complexity evaluation mechanism to detect expensive operations

[jefrajames]

GraphQL provides a lot of flexibility on the client-side which exposes to the risk of expensive operations on the server leading to potential DOS attacks. It would be nice to introduce a mechanism to limit the cost of an operation on the server.

SPQR provides with a mechanism that enables to limit the complexity of a schema and to define the cost of an operation with a @GraphQLComplexity annotation.

This is showcased in this test class:
https://github.com/leangen/graphql-spqr/blob/master/src/test/java/io/leangen/graphql/ComplexityTest.java

A similar feature would be a nice to have in the spec.

[t1]

More background about this can be found here