Support for limiting operations by field

[nadeemc]

Hi!

The goal of this PR is to be able to limit the predicates generated by field.

This was motivated by the fact that some fields, e.g. slug, should have less flexibility on how they are filtered, i.e. slug and slugIn as the only predicates instead of slugNEQ, slugNotIn, etc.

Linted using golangci-lint. Tests updated and passed for using the AllowedOps annotation on the username field.