Add mechanism to dynamically control visibility of fields and types

[ccbrown]

What it Does

This enables users to implement feature flagging as described in https://github.com/graphql/graphql-wg/blob/main/rfcs/OptInFeatures.md by defining functions on field and type definitions which control their visibility.

Steps to Test

go test -v ./...

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 18 lines in your changes are missing coverage. Please review.

Project coverage is 79.36%. Comparing base (c289cff) to head (57d1c2d).

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #68      +/-   ##
==========================================
- Coverage   79.61%   79.36%   -0.25%     
==========================================
  Files          62       62              
  Lines        5042     5060      +18     
==========================================
+ Hits         4014     4016       +2     
- Misses        681      697      +16     
  Partials      347      347              

Files	Coverage Δ
graphql/schema/field_definition.go	36.36% <ø> (ø)
graphql/schema/schema.go	77.65% <ø> (ø)
graphql/schema/enum_type.go	64.28% <0.00%> (-7.72%)	⬇️
graphql/schema/input_object_type.go	73.77% <0.00%> (-3.82%)	⬇️
graphql/schema/interface_type.go	33.33% <0.00%> (-8.34%)	⬇️
graphql/schema/object_type.go	46.51% <0.00%> (-3.49%)	⬇️
graphql/schema/scalar_type.go	82.35% <0.00%> (-17.65%)	⬇️
graphql/schema/union_type.go	38.88% <0.00%> (-7.78%)	⬇️

... and 1 file with indirect coverage changes

[ccbrown]

Abandoning in favor of a mechanism that can be better validated before execution time.