api-ref: include links on backups list and details

This patch adds backups_links optional on parameters.yaml and
include as response parameter on v3 backups list(/backups)
and details(/backups/detail).

Closes-Bug: 1944463
Change-Id: I29d2fb4b33f508638d703659a7717edf2891527d

api-ref/source/v3/ext-backups.inc

@@ -99,6 +99,7 @@ Response Parameters
    - metadata: metadata_backup
    - user_id: user_id_backup
    - encryption_key_id: encryption_key_id
+   - backup_links: backup_links_optional
 
 Response Example
 ----------------
@@ -418,6 +419,7 @@ Response Parameters
    - links: links_backup
    - name: name_backup
    - count: count
+   - backup_links: backup_links_optional
 
 Response Example
 ----------------

api-ref/source/v3/parameters.yaml

@@ -664,6 +664,16 @@ backup_id:
   required: false
   type: string
   min_version: 3.47
+backup_links_optional:
+  description: |
+    An array containing an object with the following fields:
+    ``"rel"`` with the value ``"next"`` and ``href``,
+    whose value is a link to the next page of backups.
+    Only appears when there are more backups than are
+    listed in the current response.
+  in: body
+  required: false
+  type: array
 backup_record:
   description: |
     An object recording volume backup metadata, including ``backup_service``