User data source

[dwimsey]

This is more or less a copy and paste of the service account data source, with minimal changes to support the differences for a user account.

The Full Name field from the API is referenced as display_name in the data source - I wasn't sure if it should stay as display_name to be more like other objects or if it should be named full_name to match the API - happy to change it if needed.

I have also implemented a user resource, but since they can not be created via API at this time I wasn't sure how to handle that just yet. In my implementation an error is thrown if you attempt to create a resource, the expectation being that they would be imported using terraform import - Didn't include it here as it doesn't seem like the proper way to implement the resource - the only changeable field being the full name field a data source seems all thats likely needed for most

[linouk23]

Thank you so much for opening this PR! That totally makes sense to have confluentcloud_user data source.

Unfortunately our repository isn't setup to accept PRs but

1. I've added a corresponding issue Add confluentcloud_user data source #61 to track it

2. We'll add confluentcloud_user data source in our next release which will be based on your amazing PR!