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/configuration/AwsAttachmentsConfiguration.java

@@ -19,7 +19,7 @@ package org.whispersystems.textsecuregcm.configuration;
 import com.fasterxml.jackson.annotation.JsonProperty;
 import org.hibernate.validator.constraints.NotEmpty;
 
-public class AttachmentsConfiguration {
+public class AwsAttachmentsConfiguration {
 
   @NotEmpty
   @JsonProperty

service/src/main/java/org/whispersystems/textsecuregcm/configuration/GcpAttachmentsConfiguration.java

@@ -0,0 +1,56 @@
+package org.whispersystems.textsecuregcm.configuration;
+
+import com.fasterxml.jackson.annotation.JsonProperty;
+import io.dropwizard.util.Strings;
+import io.dropwizard.validation.ValidationMethod;
+import org.hibernate.validator.constraints.NotEmpty;
+
+import javax.validation.constraints.Min;
+
+public class GcpAttachmentsConfiguration {
+
+  @NotEmpty
+  @JsonProperty
+  private String domain;
+
+  @NotEmpty
+  @JsonProperty
+  private String email;
+
+  @JsonProperty
+  @Min(1)
+  private int maxSizeInBytes;
+
+  @JsonProperty
+  private String pathPrefix;
+
+  @NotEmpty
+  @JsonProperty
+  private String rsaSigningKey;
+
+  public String getDomain() {
+    return domain;
+  }
+
+  public String getEmail() {
+    return email;
+  }
+
+  public int getMaxSizeInBytes() {
+    return maxSizeInBytes;
+  }
+
+  public String getPathPrefix() {
+    return pathPrefix;
+  }
+
+  public String getRsaSigningKey() {
+    return rsaSigningKey;
+  }
+
+  @SuppressWarnings("unused")
+  @ValidationMethod(message = "pathPrefix must be empty or start with /")
+  public boolean isPathPrefixValid() {
+    return Strings.isNullOrEmpty(pathPrefix) || pathPrefix.startsWith("/");
+  }
+}