Add coder_monitoring module and handling in terraform_provider

⚠ subissue of coder #15746

OOM & OOD Notifications require to let users set thresholds for both of these resources.

The threshold are connected to templates and injected at build time of a workspace.

In order to do so, we need to have a way, in templates, to inject new modules from terraform - similar to coder_monitoring with values that will then be parsed, and injected into the db, connected to the workspace.

The scope of this specific issue stops as soon as we are able to store the new threshold into the database and have a method to retrieve it.

Start by tracing the existing coder_monitoring module and terraform_provider handling described in the issue, then follow how template values are injected at workspace build time and persisted. The work is done when the new threshold values can be stored in the database and retrieved through a method, without implementing the later notification behavior.

Scritto dal modello di indicizzazione a partire dal testo della issue.