feat(coderd): add support for external agents to API's and provisioner (#19286)

This pull request introduces support for external workspace management, allowing users to register and manage workspaces that are provisioned and managed outside of the Coder.

Depends on: https://github.com/coder/terraform-provider-coder/pull/424

* GET /api/v2/init-script - Gets the agent initialization script
  * By default, it returns a script for Linux (amd64), but with query parameters (os and arch) you can get the init script for different platforms
* GET /api/v2/workspaces/{workspace}/external-agent/{agent}/credentials - Gets credentials for an external agent **(enterprise)**
* Updated queries to filter workspaces/templates by the has_external_agent field

coderd/templateversions.go

@@ -1963,6 +1963,7 @@ func convertTemplateVersion(version database.TemplateVersion, job codersdk.Provi
 		Archived:            version.Archived,
 		Warnings:            warnings,
 		MatchedProvisioners: matchedProvisioners,
+		HasExternalAgent:    version.HasExternalAgent.Bool,
 	}
 }