Bring over structured Swift syntax from OpenAPI (#1722)

Motivation:

We’ll need to be able to represent code to generate as structured Swift types for the new Codegen library.
This is effectively what swift-syntax provides but we can’t (at time of writing) take it as a dependency
because of its unstable API. Swift OpenAPI Generator has already provided a number of these types as its StructuedSwiftRepresentation.

Modifications:

- Copied StructuredSwiftRepresentation.swift inside the Codegen library module
- Mentioned the usage in NOTICES.txt

Result:

Codegen library will have a representation of the code to generate as structured Swift types.

NOTICES.txt

@@ -57,3 +57,12 @@ This product uses derivations of swift-extras/swift-extras-base64 'Base64.swift'
     * https://github.com/swift-extras/swift-extras-base64/blob/main/LICENSE
   * HOMEPAGE:
     * https://github.com/swift-extras/swift-extras-base64
+
+---
+
+This product uses derivations of apple/swift-openapi-generator 'StructuredSwiftRepresentation.swift'.
+    
+    * LICENSE (Apache License 2.0):
+      * https://github.com/apple/swift-openapi-generator/blob/main/LICENSE.txt
+    * HOMEPAGE:
+      * https://github.com/apple/swift-openapi-generator