Add v4 attachment controller

Add AttachmentControllerV4 which can be configured to generate upload
forms for a TUS based CDN

service/src/main/java/org/whispersystems/textsecuregcm/entities/AttachmentDescriptorV3.java

@@ -5,7 +5,22 @@
 
 package org.whispersystems.textsecuregcm.entities;
 
+import io.swagger.v3.oas.annotations.media.Schema;
 import java.util.Map;
 
-public record AttachmentDescriptorV3(int cdn, String key, Map<String, String> headers, String signedUploadLocation) {
+public record AttachmentDescriptorV3(
+    @Schema(description = """
+        Indicates the CDN type. 2 in the v3 API, 2 or 3 in the v4 API.
+        2 indicates resumable uploads using GCS,
+        3 indicates resumable uploads using TUS
+        """)
+    int cdn,
+    @Schema(description = "The location within the specified cdn where the finished upload can be found")
+    String key,
+    @Schema(description = "A map of headers to include with all upload requests. Potentially contains time-limited upload credentials")
+    Map<String, String> headers,
+
+    @Schema(description = "The URL to upload to with the appropriate protocol")
+    String signedUploadLocation) {
+
 }