feat: add a workspace preset datasource

[SasSwart]

This pull request adds data "coder_workspace_preset" {}

Basic validation is done when the data source is read, but cross validation against coder_parameter data sources happens in the provisioner in coder/coder.

Once merged into main, we'll need to backport to v1.0.5 and bump the dependency in coder/coder, because we rely on types in this provider in the coder provisioner.

[SasSwart]

Self review:

[dannykopping]

Minor comment about the idiomatic approach to validation but otherwise LGTM
Feel free to ignore the nits.