Before: the concept "Content string" is used everywhere. It has some
problems:
1. Sometimes it means "base64 encoded content", sometimes it means "raw
binary content"
2. It doesn't work with large files, eg: uploading a 1G LFS file would
make Gitea process OOM
This PR does the refactoring: use "ContentReader" / "ContentBase64"
instead of "Content"
This PR is not breaking because the key in API JSON is still "content":
`` ContentBase64 string `json:"content"` ``
Follow #25229
At present, when the trigger event is `pull_request_target`, the `ref`
and `sha` of `ActionRun` are set according to the base branch of the
pull request. This makes it impossible for us to find the head branch of
the `ActionRun` directly. In this PR, the `ref` and `sha` will always be
set to the head branch and they will be changed to the base branch when
generating the task context.
Fix#25088
This PR adds the support for
[`pull_request_target`](https://docs.github.com/en/actions/using-workflows/events-that-trigger-workflows#pull_request_target)
workflow trigger. `pull_request_target` is similar to `pull_request`,
but the workflow triggered by the `pull_request_target` event runs in
the context of the base branch of the pull request rather than the head
branch. Since the workflow from the base is considered trusted, it can
access the secrets and doesn't need approvals to run.
