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

cli/speedtest_test.go

@@ -24,9 +24,8 @@ func TestSpeedtest(t *testing.T) {
 	agentClient := codersdk.New(client.URL)
 	agentClient.SessionToken = agentToken
 	agentCloser := agent.New(agent.Options{
-		FetchMetadata:     agentClient.WorkspaceAgentMetadata,
-		CoordinatorDialer: agentClient.ListenWorkspaceAgentTailnet,
-		Logger:            slogtest.Make(t, nil).Named("agent"),
+		Client: agentClient,
+		Logger: slogtest.Make(t, nil).Named("agent"),
 	})
 	defer agentCloser.Close()
 	coderdtest.AwaitWorkspaceAgents(t, client, workspace.ID)