add user rename endpoint to admin api (#22789)

this is a simple endpoint that adds the ability to rename users to the
admin API.

Note: this is not in a mergeable state. It would be better if this was
handled by a PATCH/POST to the /api/v1/admin/users/{username} endpoint
and the username is modified.

---------

Co-authored-by: Jason Song <i@wolfogre.com>

templates/swagger/v1_json.tmpl

@@ -679,6 +679,46 @@
         }
       }
     },
+    "/admin/users/{username}/rename": {
+      "post": {
+        "produces": [
+          "application/json"
+        ],
+        "tags": [
+          "admin"
+        ],
+        "summary": "Rename a user",
+        "operationId": "adminRenameUser",
+        "parameters": [
+          {
+            "type": "string",
+            "description": "existing username of user",
+            "name": "username",
+            "in": "path",
+            "required": true
+          },
+          {
+            "name": "body",
+            "in": "body",
+            "required": true,
+            "schema": {
+              "$ref": "#/definitions/RenameUserOption"
+            }
+          }
+        ],
+        "responses": {
+          "204": {
+            "$ref": "#/responses/empty"
+          },
+          "403": {
+            "$ref": "#/responses/forbidden"
+          },
+          "422": {
+            "$ref": "#/responses/validationError"
+          }
+        }
+      }
+    },
     "/admin/users/{username}/repos": {
       "post": {
         "consumes": [
@@ -19105,6 +19145,22 @@
       },
       "x-go-package": "code.gitea.io/gitea/modules/structs"
     },
+    "RenameUserOption": {
+      "description": "RenameUserOption options when renaming a user",
+      "type": "object",
+      "required": [
+        "new_username"
+      ],
+      "properties": {
+        "new_username": {
+          "description": "New username for this user. This name cannot be in use yet by any other user.",
+          "type": "string",
+          "uniqueItems": true,
+          "x-go-name": "NewName"
+        }
+      },
+      "x-go-package": "code.gitea.io/gitea/modules/structs"
+    },
     "RepoCollaboratorPermission": {
       "description": "RepoCollaboratorPermission to get repository permission for a collaborator",
       "type": "object",