Add OpenAPI spec for safekeeper HTTP API (neondatabase/cloud#1264, #2061)

This spec is used in the `cloud` repo to generate HTTP client.

.github/ansible/scripts/init_safekeeper.sh

@@ -12,10 +12,9 @@ cat <<EOF | tee /tmp/payload
   "version": 1,
   "host": "${HOST}",
   "port": 6500,
+  "http_port": 7676,
   "region_id": {{ console_region_id }},
-  "instance_id": "${INSTANCE_ID}",
-  "http_host": "${HOST}",
-  "http_port": 7676
+  "instance_id": "${INSTANCE_ID}"
 }
 EOF