feat: add TypeSystem directives support with introspection

[jwaldrip]

Summary

This adds full TypeSystem directive support as requested in issue #1003.

Changes

• Add applied_directives field to all Type structs (Object, Scalar, Field, Interface, Union, Enum, InputObject, Argument, Enum.Value)
• Preserve applied directives through the build phase from Blueprint to final Type structs
• Add __AppliedDirective and __DirectiveArgument introspection types
• Add appliedDirectives field to __Type, __Field, __InputValue, and __EnumValue introspection types

Supported Locations

TypeSystem directives can be applied to all spec-defined locations:

• SCHEMA, SCALAR, OBJECT, FIELD_DEFINITION, ARGUMENT_DEFINITION
• INTERFACE, UNION, ENUM, ENUM_VALUE, INPUT_OBJECT, INPUT_FIELD_DEFINITION

The directive expand callback continues to work for transforming type definitions at compile time, and applied directives are now visible through introspection queries.

Example Introspection

{
  __type(name: "User") {
    appliedDirectives {
      name
      args {
        name
        value
      }
    }
  }
}

Closes #1003

Test plan

• Added comprehensive TypeSystem directive tests (25 new tests)
• All 1467 tests pass
• Tested directive application on all type system elements
• Verified introspection returns applied directives correctly

🤖 Generated with Claude Code

[jwaldrip]

Any thoughts on the TypeSystem directives support? This adds introspection for directives applied to schema types. Would love any feedback or concerns.

[cschiewek]

I'm in favour of adding the applied_directives field to all the type structs, but definitely against the new introspection types as they are not part of the spec. IMO, if folks want to extend the introspection schema, it should be a seperate package / plugin.

[jwaldrip]

@cschiewek Good call — removed the __AppliedDirective and __DirectiveArgument introspection types and the appliedDirectives fields from __type, __field, __inputvalue, and __enumvalue. Agreed these shouldn't extend the introspection schema beyond the spec.

The applied_directives field on the type structs (runtime) is kept so directives applied to types/fields/args are still accessible programmatically via Absinthe.Schema.lookup_type/2 and friends.