chore: refactor instance identity to be a SessionTokenProvider (#19566)

Refactors Agent instance identity to be a SessionTokenProvider.

Refactors the CLI to create Agent clients via a centralized function, rather than add-hoc via individual command handlers and their flags.

This allows commands besides `coder agent`, but which still use the agent identity, to support instance identity authentication.

Fixes #19111 by unifying all API requests to go thru the SessionTokenProvider for auth credentials.

coderd/workspaceagentsrpc_test.go

@@ -68,8 +68,7 @@ func TestWorkspaceAgentReportStats(t *testing.T) {
 				},
 			).Do()
 
-			ac := agentsdk.New(client.URL)
-			ac.SetSessionToken(r.AgentToken)
+			ac := agentsdk.New(client.URL, agentsdk.WithFixedToken(r.AgentToken))
 			conn, err := ac.ConnectRPC(context.Background())
 			require.NoError(t, err)
 			defer func() {
@@ -155,8 +154,7 @@ func TestAgentAPI_LargeManifest(t *testing.T) {
 				agents[0].ApiKeyScope = string(tc.apiKeyScope)
 				return agents
 			}).Do()
-			ac := agentsdk.New(client.URL)
-			ac.SetSessionToken(r.AgentToken)
+			ac := agentsdk.New(client.URL, agentsdk.WithFixedToken(r.AgentToken))
 			conn, err := ac.ConnectRPC(ctx)
 			defer func() {
 				_ = conn.Close()