API spec: add safekeepers field returned by storcon (#11385)

Add an optional `safekeepers` field to `TimelineInfo` which is returned
by the storcon upon timeline creation if the
`--timelines-onto-safekeepers` flag is enabled. It contains the list of
safekeepers chosen.

Other contexts where we return `TimelineInfo` do not contain the
`safekeepers` field, sadly I couldn't make this more type safe like done
in Rust via `TimelineCreateResponseStorcon`, as there is no way of
flattening or inheritance (and I don't that duplicating the entire type
for some minor type safety improvements is worth it).

The storcon side has been done in #11058.

Part of https://github.com/neondatabase/cloud/issues/16176
cc https://github.com/neondatabase/cloud/issues/16796

pageserver/src/http/openapi_spec.yml

@@ -1133,6 +1133,40 @@ components:
         applied_gc_cutoff_lsn:
           type: string
           format: hex
+        safekeepers:
+          $ref: "#/components/schemas/TimelineSafekeepersInfo"
+
+    TimelineSafekeepersInfo:
+      type: object
+      required:
+        - tenant_id
+        - timeline_id
+        - generation
+        - safekeepers
+      properties:
+        tenant_id:
+          type: string
+          format: hex
+        timeline_id:
+          type: string
+          format: hex
+        generation:
+          type: integer
+        safekeepers:
+          type: array
+          items:
+            $ref: "#/components/schemas/TimelineSafekeeperInfo"
+
+    TimelineSafekeeperInfo:
+      type: object
+      required:
+        - id
+        - hostname
+      properties:
+        id:
+          type: integer
+        hostname:
+          type: string
 
     SyntheticSizeResponse:
       type: object