Add API to query collaborators permission for a repository (#18761)

Targeting #14936, #15332

Adds a collaborator permissions API endpoint according to GitHub API: https://docs.github.com/en/rest/collaborators/collaborators#get-repository-permissions-for-a-user to retrieve a collaborators permissions for a specific repository.

### Checks the repository permissions of a collaborator. 

`GET` `/repos/{owner}/{repo}/collaborators/{collaborator}/permission`

Possible `permission` values are `admin`, `write`, `read`, `owner`, `none`.

```json
{
  "permission": "admin",
  "role_name": "admin",
  "user": {}
}
```

Where `permission` and `role_name` hold the same `permission` value and `user` is filled with the user API object. Only admins are allowed to use this API endpoint.

templates/swagger/v1_json.tmpl

@@ -3129,6 +3129,52 @@
         }
       }
     },
+    "/repos/{owner}/{repo}/collaborators/{collaborator}/permission": {
+      "get": {
+        "produces": [
+          "application/json"
+        ],
+        "tags": [
+          "repository"
+        ],
+        "summary": "Get repository permissions for a user",
+        "operationId": "repoGetRepoPermissions",
+        "parameters": [
+          {
+            "type": "string",
+            "description": "owner of the repo",
+            "name": "owner",
+            "in": "path",
+            "required": true
+          },
+          {
+            "type": "string",
+            "description": "name of the repo",
+            "name": "repo",
+            "in": "path",
+            "required": true
+          },
+          {
+            "type": "string",
+            "description": "username of the collaborator",
+            "name": "collaborator",
+            "in": "path",
+            "required": true
+          }
+        ],
+        "responses": {
+          "200": {
+            "$ref": "#/responses/RepoCollaboratorPermission"
+          },
+          "403": {
+            "$ref": "#/responses/forbidden"
+          },
+          "404": {
+            "$ref": "#/responses/notFound"
+          }
+        }
+      }
+    },
     "/repos/{owner}/{repo}/commits": {
       "get": {
         "produces": [
@@ -17451,6 +17497,24 @@
       },
       "x-go-package": "code.gitea.io/gitea/modules/structs"
     },
+    "RepoCollaboratorPermission": {
+      "description": "RepoCollaboratorPermission to get repository permission for a collaborator",
+      "type": "object",
+      "properties": {
+        "permission": {
+          "type": "string",
+          "x-go-name": "Permission"
+        },
+        "role_name": {
+          "type": "string",
+          "x-go-name": "RoleName"
+        },
+        "user": {
+          "$ref": "#/definitions/User"
+        }
+      },
+      "x-go-package": "code.gitea.io/gitea/modules/structs"
+    },
     "RepoCommit": {
       "type": "object",
       "title": "RepoCommit contains information of a commit in the context of a repository.",
@@ -19126,6 +19190,12 @@
         }
       }
     },
+    "RepoCollaboratorPermission": {
+      "description": "RepoCollaboratorPermission",
+      "schema": {
+        "$ref": "#/definitions/RepoCollaboratorPermission"
+      }
+    },
     "Repository": {
       "description": "Repository",
       "schema": {