Add API for Act Runner's Variables Management  

### Feature Description

It would be great if there is an API to create/change/delete the variables used by the runner in a repo.

Currently, an action can only read variables. There is no way to modify variables.
[
https://docs.gitea.com/next/usage/actions/act-runner#using-variable](
https://docs.gitea.com/next/usage/actions/act-runner#using-variable)

### Screenshots

_No response_