Create attachments V3 endpoint for CDN2 on GCP

In preparation for resumable uploads, this creates a separate
attachment authorization endpoint that creates a signed URL for
accessing GCP Storage through Signal's CDN2. This should allow Signal
clients to do byte-level resume of media uploads.

service/src/main/java/org/whispersystems/textsecuregcm/s3/PolicySigner.java

@@ -1,7 +1,6 @@
 package org.whispersystems.textsecuregcm.s3;
 
 import com.amazonaws.util.Base16Lower;
-import com.google.common.annotations.VisibleForTesting;
 
 import javax.crypto.Mac;
 import javax.crypto.spec.SecretKeySpec;