[PHP] Support for untyped Json

Similar to https://github.com/microsoft/kiota/pull/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 https://github.com/microsoft/kiota-abstractions-dotnet/pull/177)
- [ ] Add derived types of `UntypedNode` to represent object, array and primitive types in the abstractions (see https://github.com/microsoft/kiota-abstractions-dotnet/pull/177)
- [ ] Update the `getObjectValue` method in the json serialization library to handle scenarios of untyped content (see https://github.com/microsoft/kiota-serialization-json-dotnet/pull/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.

Start by reviewing the linked Kiota .NET abstraction and JSON serialization pull requests, then locate getObjectValue and RemoveUntypedNodePropertyValues in the PHP generation and serialization flow. Use the issue checklist to identify the required abstraction types, generator tests, and integration coverage. Done means untyped object, array, and primitive content is preserved and the apisguru::apis.guru integration test passes.

Written by the indexing model from the issue text.