[PHP] Support for untyped Json

[andrueastman]

Similar to #4095 we need to add support for untyped content in request/response payloads.

In summary, we need to

• Add UntypedNode base type in the abstractions library (see Create types for untyped nodes kiota-dotnet#177)
• Add derived types of UntypedNode to represent object, array and primitive types in the abstractions (see Create types for untyped nodes kiota-dotnet#177)
• Update the getObjectValue method in the json serialization library to handle scenarios of untyped content (see Support untyped nodes kiota-serialization-json-dotnet#198)
• Remove the call to RemoveUntypedNodePropertyValues in the refiner so that untyped properties are not stripped out.
• Add tests in the generator.
• Ensure the "apisguru::apis.guru" integration test is passing.