feat: add coderd_user data source

[ethanndickson]

An acceptance test is written, but commented out until we decide how we'll run it against a coderd instance.

Relates to #2

[coadler]

We should probably remove this or change it to Coder

[ethanndickson]

I'm not super confident in the language used by the MPL2:

3.4. Notices
You may not remove or alter the substance of any license notices
(including copyright notices, patent notices, disclaimers of warranty,
or limitations of liability) contained within the Source Code Form of
the Covered Software, except that You may alter any license notices to
the extent required to remedy known factual inaccuracies.

I would assume this only applies to files we use from the framework unchanged? i.e. This is in an example data source file, but we might not use the example files and therefore not have the notice.

[ethanndickson]

I'm gonna remove it from the files where it's all our code anyway