Add CodingKeys support to @Schemable macro

[ecito]

Description

This PR adds support for Swift's CodingKeys enum to the @Schemable macro. When a type defines a CodingKeys enum with custom string raw values, the macro will automatically use those values as property names in the generated JSON schema.

Motivation

Currently, if you want different property names in your JSON schema than your Swift property names, you need to manually add @SchemaOptions(.key("custom_name")) to each property. This is repetitive when you already have a CodingKeys enum defined for Codable conformance.

Example Usage

Before:

@Schemable
struct User: Codable {
  @SchemaOptions(.key("first_name"))
  let firstName: String
  @SchemaOptions(.key("last_name"))
  let lastName: String
  @SchemaOptions(.key("email"))
  let emailAddress: String

  enum CodingKeys: String, CodingKey {
    case firstName = "first_name"
    case lastName = "last_name"
    case emailAddress = "email"
  }
}

After:

@Schemable
struct User: Codable {
  let firstName: String
  let lastName: String
  let emailAddress: String

  enum CodingKeys: String, CodingKey {
    case firstName = "first_name"
    case lastName = "last_name"
    case emailAddress = "email"
  }
}

Both generate the same schema with properties: "first_name", "last_name", "email"

Key Features

• Automatic mapping: Extracts key mappings from existing CodingKeys enum
• Partial support: Properties without CodingKeys entries fall back to property names
• Priority system: @SchemaOptions(.key) > CodingKeys > keyStrategy > property name
• Nested types: Each type's CodingKeys is properly scoped
• Backward compatible: No breaking changes, purely additive

Additional Examples

Partial CodingKeys:

@Schemable
struct Product {
  let name: String           // Uses "name" (no CodingKeys entry)
  let productId: Int         // Uses "product_id" (from CodingKeys)

  enum CodingKeys: String, CodingKey {
    case productId = "product_id"
  }
}

Priority Override:

@Schemable
struct Customer {
  let firstName: String      // Uses "first_name" (from CodingKeys)
  @SchemaOptions(.key("surname"))
  let lastName: String       // Uses "surname" (@SchemaOptions takes priority)

  enum CodingKeys: String, CodingKey {
    case firstName = "first_name"
    case lastName = "last_name"  // Overridden by @SchemaOptions
  }
}

Type of Change

• Bug fix
• New feature
• Breaking change
• Documentation update

Additional Notes

• All existing tests continue to pass
• Added comprehensive integration tests for CodingKeys support
• Added macro expansion tests to verify generated code
• Implementation uses SwiftSyntax AST parsing for robustness

[ajevans99]

Like the idea. Let's just update to macros docc page key coding section https://swiftpackageindex.com/ajevans99/swift-json-schema/main/documentation/jsonschemabuilder/macros#Key-encoding-strategies and we're good to go