feat: Add directive layer

[michael-georgiadis]

Description

As per your comment on this PR, I am splitting the implementation into two.

I am focusing on configuring the directive layer on this one. Specifying two directives to start with.

How it works

A directive is a PHP attribute implementing one of the family interfaces, which ties it to a GraphQL location (FieldDirective → FIELD_DEFINITION, InputObjectTypeDirective → INPUT_OBJECT, etc.). Implementing the marker interface alone is metadata-only; implementing the matching Behavioral* sub-interface adds an apply hook that runs through a middleware pipe during type generation.

The built-ins on the layer

• #[OneOf] → sets webonyx's isOneOf flag → prints input LookupInput @oneOf.
• #[Deprecated(reason:)] → sets the field's deprecation reason → prints
  legacy: String! @deprecated(reason: "...").

Both bind to directives webonyx already declares, so they print through webonyx's own SDL output and we don't register duplicate definitions for them (DirectiveDefinition::$builtIn).

@deprecated: additive, not a rewrite

The existing docblock @deprecated support is untouched. #[Deprecated] is an attribute-based
path that composes with it rather than fighting it:

Declaration	Result
#[Deprecated(reason: 'X')] (± docblock)	@deprecated(reason: "X") — explicit wins
#[Deprecated] + /** @deprecated Y */	@deprecated(reason: "Y") — docblock kept
#[Deprecated], no docblock	@deprecated(reason: "No longer supported") — default

Usage validation

PHP's #[Attribute] targets can't tell a #[Type] class from an #[Input] class (both are TARGET_CLASS), so a misplaced #[OneOf] on a #[Type] would otherwise be silently dropped by the interface-based collection. DirectiveValidator catches that and throws a clear InvalidDirectiveException instead.