Python: Support `document` type (#2188)

* Add Python wrapper for `aws_smithy_types::Document`

* Remove unused type

* Make sure Python SSDKs uses Python specific `Document` type

* Allow Python specific `Document` type to be used in serialization and deserialization

* Make `pyo3/extension-module` a default feature

* Add `PythonServerTypesTest`

* Fix linting issues