changelog submit: provide ability to push to fork PRs when allowed to

Most of the PRs directly opened by humans come from forks, at least this is the case of https://github.com/elastic/elastic-otel-java.
Unless it's for very specific purpose like working on github actions, we can assume this will be the case for all PRs.

When there is no changelog, this means that the current changelog submit will issue a comment to add the changelog to the PR, even if the PR author grants the repository owners the permission to modify the PR.

One common use-case for this is to being able to update PRs of community contributions to push changes to changelog without depending on the PR author action (it's very probably the default).

It would be nice if we could have the submit task able to do this, I have no idea if the credentials used to push to internal branches are considered "repository owner" in this context nor if there is a way to query the state of the PR before deciding on the ability to push to it or issue a comment.