fix: Refactor agent to consume API client (#4715)

* fix: Refactor agent to consume API client

This simplifies a lot of code by creating an interface for
the codersdk client into the agent. It also moves agent
authentication code so instance identity will work between
restarts.

Fixes #3485 and #4082.

* Fix client reconnections

coderd/templates_test.go

@@ -603,10 +603,8 @@ func TestTemplateMetrics(t *testing.T) {
 	agentClient := codersdk.New(client.URL)
 	agentClient.SessionToken = authToken
 	agentCloser := agent.New(agent.Options{
-		Logger:            slogtest.Make(t, nil),
-		StatsReporter:     agentClient.AgentReportStats,
-		FetchMetadata:     agentClient.WorkspaceAgentMetadata,
-		CoordinatorDialer: agentClient.ListenWorkspaceAgentTailnet,
+		Logger: slogtest.Make(t, nil),
+		Client: agentClient,
 	})
 	defer func() {
 		_ = agentCloser.Close()