Support modifying the default operation names

Because we may be burdened with domain models that are either poorly or lengthily named, it would be helpful to have the ability to provide custom operation names in the GraphQLMapping DSL.

I've already actually implemented this, just need to submit a PR for it.

Start by reviewing the GraphQLMapping DSL and the existing implementation mentioned in the issue. Confirm how custom operation names should be exposed and documented, then submit a pull request demonstrating the completed behavior; no specific files or tests are named in the issue.

Written by the indexing model from the issue text.