Redact all secrets from config viewing APIs (#17380)

This change adds a `Secret` property to `HelpKV` to identify secrets
like passwords and auth tokens that should not be revealed by the server
in its configuration fetching APIs. Configuration reporting APIs now do
not return secrets.

internal/config/lambda/help.go

@@ -37,6 +37,7 @@ var (
 			Optional:    true,
 			Type:        "string",
 			Sensitive:   true,
+			Secret:      true,
 		},
 		config.HelpKV{
 			Key:         config.Comment,