TUN-8914: Create a flags module to group all cloudflared cli flags

## Summary

This commit refactors some of the flags of cloudflared to their own module, so that they can be used across the code without requiring to literal strings which are much more error prone.

 Closes TUN-8914

credentials/origin_cert.go

@@ -15,7 +15,6 @@ import (
 
 const (
 	DefaultCredentialFile = "cert.pem"
-	OriginCertFlag        = "origincert"
 )
 
 type namedTunnelToken struct {