admin: added definition for topic mount/unmount APIs

Added definitions for topic mount/unmount REST APIs. The definitions are
placed in `migrations.json` file which is counter intuitive but it
allows reusing the data migration related definitions.

Signed-off-by: Michał Maślanka <michal@redpanda.com>

src/v/redpanda/admin/api-doc/migration.def.json

@@ -147,4 +147,27 @@
             "type": "inbound_migration"
         }
     }
+},
+"mount_configuration": {
+    "type": "object",
+    "required": [
+        "topics"
+    ],
+    "properties": {
+        "topics": {
+            "type": "array",
+            "items": {
+                "type": "inbound_topic"
+            },
+            "description": "List of topics to mount"
+        }
+    }
+},
+"migration_info": {
+    "type": "object",
+    "properties": {
+        "id": {
+            "type": "int"
+        }
+    }
 }

src/v/redpanda/admin/api-doc/migration.json

@@ -89,4 +89,51 @@
             }
         ]
     }
+},
+"/v1/topics/mount": {
+    "post": {
+        "operationId": "mount_topics",
+        "summary": "Mount topic according to provided configuration",
+        "requestBody": {
+            "description": "Mount topic configuration",
+            "required": true,
+            "content": {
+                "application/json": {
+                    "schema": "mount_configuration"
+                }
+            }
+        },
+        "responses": {
+            "200": {
+                "description": "Underlying migration information",
+                "schema": "migration_info"
+            }
+        }
+    }
+},
+"/v1/topics/unmount": {
+    "post": {
+        "operationId": "unmount_topics",
+        "summary": "Unmount the topics provided in the list",
+        "requestBody": {
+            "description": "List of topics to unmount",
+            "required": true,
+            "content": {
+                "application/json": {
+                    "schema": {
+                        "type": "array",
+                        "items": {
+                            "type": "namespaced_topic"
+                        }
+                    }
+                }
+            }
+        },
+        "responses": {
+            "200": {
+                "description": "Underlying migration information",
+                "schema": "migration_info"
+            }
+        }
+    }
 }