Add `coder_template` data source

[matifali]

As suggested in https://github.com/coder/coder/discussions/13122

It would be nice to have a data source coder_template to read template properties.

1. data.coder_workspace.me.template_id
2. data.coder_workspace.me.template_name}
3. data.coder_workspace.me.template_version

etc.

This and #219 can be added to the coder provider or a new provider to manage Coder deployment and templates with infrastructure as code.