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.

2 files changed, 16 insertions, 0 deletions

diff --git a/modules/convert/user.go b/modules/convert/user.go
index dc4a8c49c7..2b07d21838 100644
--- a/modules/convert/user.go
+++ b/modules/convert/user.go
@@ -95,3 +95,12 @@ func User2UserSettings(user *user_model.User) api.UserSettings {
 		DiffViewStyle: user.DiffViewStyle,
 	}
 }
+
+// ToUserAndPermission return User and its collaboration permission for a repository
+func ToUserAndPermission(user, doer *user_model.User, accessMode perm.AccessMode) api.RepoCollaboratorPermission {
+	return api.RepoCollaboratorPermission{
+		User:       ToUser(user, doer),
+		Permission: accessMode.String(),
+		RoleName:   accessMode.String(),
+	}
+}
diff --git a/modules/structs/repo_collaborator.go b/modules/structs/repo_collaborator.go
index 2b4fa390d2..2f9c8992a1 100644
--- a/modules/structs/repo_collaborator.go
+++ b/modules/structs/repo_collaborator.go
@@ -8,3 +8,10 @@ package structs
 type AddCollaboratorOption struct {
 	Permission *string `json:"permission"`
 }
+
+// RepoCollaboratorPermission to get repository permission for a collaborator
+type RepoCollaboratorPermission struct {
+	Permission string `json:"permission"`
+	RoleName   string `json:"role_name"`
+	User       *User  `json:"user"`
+}