Add `coder_user` data source

[matifali]

Extracted from https://github.com/coder/envbuilder/pull/174#discussion_r1591093964

The properties that are per user and not per workspace should be added/refactored to the new data source, coder_user.

Other properties that should be added or moved from coder_workspace can be

1. coder_workspace.owner -> coder_user.name
2. coder_workspace.owner_name -> coder_user.full_name
3. coder_workspace.owner_emial -> coder_user.email
4. Add coder_user.private_ssh_key
5. Add coder_user.public_ssh_key
6. Add coder_user.groups (A list of all groups if applicable)