Move PGP object factory code into a dedicated Utils class

Ghostryde.java has a lot of duplicate code with RydeEncoder and the future
RydeDecoder - the encryption/decryption, compression/decompression, file
encoding/decoding. The "de-XXX" part of each of these pairs needs to read a PGP
object from a stream using PGPObjectFactory.

Since we want to move the duplicate code into their own files, we will need to
move the "read PGP objects from stream" functions to a common utility class.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=205092800

javatests/google/registry/rde/GhostrydeTest.java

@@ -229,7 +229,8 @@ public class GhostrydeTest {
             });
     assertThat(thrown)
         .hasMessageThat()
-        .contains("Message was encrypted for keyid a59c132f3589a1d5 but ours is c9598c84ec70b9fd");
+        .contains(
+            "Message was encrypted for keyids [a59c132f3589a1d5] but ours is c9598c84ec70b9fd");
   }
 
   @Test