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.

cli/gitssh_test.go

@@ -54,8 +54,7 @@ func prepareTestGitSSH(ctx context.Context, t *testing.T) (*agentsdk.Client, str
 	}).WithAgent().Do()
 
 	// start workspace agent
-	agentClient := agentsdk.New(client.URL)
-	agentClient.SetSessionToken(r.AgentToken)
+	agentClient := agentsdk.New(client.URL, agentsdk.WithFixedToken(r.AgentToken))
 	_ = agenttest.New(t, client.URL, r.AgentToken, func(o *agent.Options) {
 		o.Client = agentClient
 	})