Add 'allow_maintainer_edit' API option for creating a pull request (#36283)

WebUI has a checkbox for enabling maintainer edits you can check right
away when creating a new pull request.
Also, it is possible to set `allow_maintainer_edit` in an existing pull
request via API.
This change enables the option while creating a new pull request via
API.

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Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

templates/swagger/v1_json.tmpl

@@ -23420,6 +23420,11 @@
       "description": "CreatePullRequestOption options when creating a pull request",
       "type": "object",
       "properties": {
+        "allow_maintainer_edit": {
+          "description": "Whether maintainers can edit the pull request",
+          "type": "boolean",
+          "x-go-name": "AllowMaintainerEdit"
+        },
         "assignee": {
           "description": "The primary assignee username",
           "type": "string",