feat: Add AWS instance identity authentication (#570)

* feat: Add AWS instance identity authentication

This allows zero-trust authentication for all AWS instances.

Prior to this, AWS instances could be used by passing `CODER_TOKEN`
as an environment variable to the startup script. AWS explicitly
states that secrets should not be passed in startup scripts because
it's user-readable.

* Fix sha256 verbosity

* Fix HTTP client being exposed on auth

cli/cliui/prompt.go

@@ -36,7 +36,7 @@ func Prompt(cmd *cobra.Command, opts PromptOptions) (string, error) {
 	signal.Notify(interrupt, os.Interrupt)
 	defer signal.Stop(interrupt)
 
-	errCh := make(chan error)
+	errCh := make(chan error, 1)
 	lineCh := make(chan string)
 	go func() {
 		var line string