cert: add TLS reloader and return bytes from GenCert

Co-authored-by: gkrizek <graham@krizek.io>

cert/go.mod

@@ -2,4 +2,7 @@ module github.com/lightningnetwork/lnd/cert
 
 go 1.16
 
-require github.com/stretchr/testify v1.5.1
+require (
+	github.com/davecgh/go-spew v1.1.1 // indirect
+	github.com/stretchr/testify v1.5.1
+)