allow graphql types and fields to be gated behind feature flags

[ccbrown]

What it Does

This adds an API for specifying a set of required features for types and object and interface fields.

The set of enabled features is given as a parameter to GraphQL validation and execution.

Types and fields that require features which are not enabled effectively do not exist – queries containing them will not validate and they will not appear in introspection.

At schema compilation time, a set of rules is used to ensure that all possible combinations of enabled feature flags result in a perfectly valid schema:

• Fields on interface implementations must require a subset of the features required by the corresponding interface field.
• Objects and interfaces must always contain at least one field whose feature requirements are a subset of their own.
• Field types must never require features that aren't required by the field itself.
• Field argument types must never require features that aren't required by the field itself.

This functionality is closely related to the proposal in the GraphQL RFC for Opt-in features and discussion happening in graphql/graphql-spec#943.

Limitations:

This PR doesn't support:

• Union members which require features not required by the union.
• Feature flagged input object fields.
• Feature flagged field arguments.
• Feature flagged enum values.

These may be added in a future PR.

Steps to Test

go test -v ./...

[codecov]

Codecov Report

Attention: Patch coverage is 76.13636% with 21 lines in your changes are missing coverage. Please review.

Project coverage is 79.85%. Comparing base (c289cff) to head (9b98469).

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #69      +/-   ##
==========================================
+ Coverage   79.61%   79.85%   +0.24%     
==========================================
  Files          62       63       +1     
  Lines        5042     5088      +46     
==========================================
+ Hits         4014     4063      +49     
+ Misses        681      680       -1     
+ Partials      347      345       -2     

Files	Coverage Δ
api.go	84.67% <100.00%> (+5.16%)	⬆️
cmd/gql-client-gen/main.go	64.56% <100.00%> (ø)
config.go	83.75% <ø> (ø)
graphql/executor/executor.go	71.14% <100.00%> (+0.24%)	⬆️
graphql/schema/enum_type.go	73.07% <100.00%> (+1.07%)	⬆️
graphql/schema/feature_set.go	100.00% <100.00%> (ø)
graphql/schema/introspection/introspection.go	93.19% <100.00%> (-0.04%)	⬇️
graphql/schema/list_type.go	79.06% <100.00%> (+0.49%)	⬆️
graphql/schema/nonnull_type.go	91.30% <100.00%> (+0.39%)	⬆️
graphql/schema/scalar_type.go	100.00% <100.00%> (ø)
... and 20 more

... and 1 file with indirect coverage changes