Add "Discover branches from forks" action to the plugin.

In some cases the workflow is such that developers develop in their own forks and only make PRs to the main repository. In such cases we can only build after a PR has been made. This might lead to a situation where developers are forced to make work-in-progress PRs to get some tests or else run in the CI.

Adding "Discover branches from forks" action to the plugin makes it possible to combine it with Named branches and Regular expression to build branches the developer wants to be built. The "Trust" choice could be used here similarly to the PRs.

This is as safe as building PRs since it would not be on by default and similar defensive measures could be used as can be used with PR building.