feat(coderd/healthcheck): add health check for proxy (#10846)

Adds a health check for workspace proxies:
- Healthy iff all proxies are healthy and the same version,
- Warning if some proxies are unhealthy,
- Error if all proxies are unhealthy, or do not all have the same version.

enterprise/coderd/workspaceproxy.go

@@ -960,3 +960,20 @@ func convertProxy(p database.WorkspaceProxy, status proxyhealth.ProxyStatus) cod
 		},
 	}
 }
+
+// workspaceProxiesFetchUpdater implements healthcheck.WorkspaceProxyFetchUpdater
+// in an actually useful and meaningful way.
+type workspaceProxiesFetchUpdater struct {
+	fetchFunc  func(context.Context) (codersdk.RegionsResponse[codersdk.WorkspaceProxy], error)
+	updateFunc func(context.Context) error
+}
+
+func (w *workspaceProxiesFetchUpdater) Fetch(ctx context.Context) (codersdk.RegionsResponse[codersdk.WorkspaceProxy], error) {
+	//nolint:gocritic // Need perms to read all workspace proxies.
+	authCtx := dbauthz.AsSystemRestricted(ctx)
+	return w.fetchFunc(authCtx)
+}
+
+func (w *workspaceProxiesFetchUpdater) Update(ctx context.Context) error {
+	return w.updateFunc(ctx)
+}