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/entitlements/entitlements.go

@@ -161,3 +161,9 @@ func (l *Set) Errors() []string {
 	defer l.entitlementsMu.RUnlock()
 	return slices.Clone(l.entitlements.Errors)
 }
+
+func (l *Set) HasLicense() bool {
+	l.entitlementsMu.RLock()
+	defer l.entitlementsMu.RUnlock()
+	return l.entitlements.HasLicense
+}