Usercloudssdk

- Introduce UCHttpClient to assist in custom HTTP clients by defining the interface in which UserClouds makes requests to [httpx](https://www.python-httpx.org/)
- Changing POST body format for authorization Create functions, and adding if_not_exists option to CreateOrganization. These are non-breaking changes, but older clients using the previous format will be deprecated eventually.

- Cleanup httpx client usage.
This is a breaking change to the HTTP client interface since we stopped passing the deprecated `data` argument to the httpx client methods (PUT & POST) and pass the `content` argument instead.
- Add more type annotations to models and add a couple of `__str__` methods to models.
- Add `include_example` argument to `DownloadUserstoreSDK` method.

- Bring back passing kwargs to the HTTP client methods.

- Add CreateUserWithMutator method to Python SDK Client and example for how to use in userstore_sample.py

- Fix future compatibility (make sure the SDK doesn't break if server sends new fields).
- Fix handling HTTP 404 responses to some HTTP DELETE API calls.
- Add support for output_type and reuse_existing_token fields in transformers.
- Lazily request access token when needed instead of on client creation.
- Improve HTTP error handling in SDK

- Added a changelog
- Switched to using [httpx](https://www.python-httpx.org/) for HTTP requests instead of [requests](https://requests.readthedocs.io/en/master/).
- Allow overriding the default HTTP client with a custom one.
- Add SDK methods for managing retention durations for soft-deleted data.
- Various other cleanup to the code.
- Method in the new Python SDK Client for downloading the custom userstore sdk for your userstore (DownloadUserstoreSDK).
- Userstore SDK now includes methods like UpdateUserForPurposes which allows you to pass the purposes in as an array of enum constants.