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/entities/AttachmentDescriptorV3.java

@@ -0,0 +1,46 @@
+package org.whispersystems.textsecuregcm.entities;
+
+import com.fasterxml.jackson.annotation.JsonProperty;
+
+import java.util.Map;
+
+public class AttachmentDescriptorV3 {
+
+  @JsonProperty
+  private int cdn;
+
+  @JsonProperty
+  private String key;
+
+  @JsonProperty
+  private Map<String, String> headers;
+
+  @JsonProperty
+  private String signedUploadLocation;
+
+  public AttachmentDescriptorV3() {
+  }
+
+  public AttachmentDescriptorV3(int cdn, String key, Map<String, String> headers, String signedUploadLocation) {
+    this.cdn                  = cdn;
+    this.key                  = key;
+    this.headers              = headers;
+    this.signedUploadLocation = signedUploadLocation;
+  }
+
+  public int getCdn() {
+    return cdn;
+  }
+
+  public String getKey() {
+    return key;
+  }
+
+  public Map<String, String> getHeaders() {
+    return headers;
+  }
+
+  public String getSignedUploadLocation() {
+    return signedUploadLocation;
+  }
+}