Extract common server encryption logic (#65)

* refactor: extract common message encryption logic
* refactor: move letsencrypt logic to common
* refactor: rename common package to encryption
* test: add encryption tests

encryption/testprotos/testproto.proto

@@ -0,0 +1,9 @@
+syntax = "proto3";
+
+option go_package = "/testprotos";
+
+package testprotos;
+
+message TestMessage {
+  string body = 1;
+}