Arbitrary script as a filter

The regexp based filter is very powerful and can be used to accomplish almost anything, but as soon as you have any kind of complexity in terms of events you want to handle, it can very quickly become unmaintainable.

For example we have a use case to trigger the same job from the following:

- PR opened against repo A
- PR merged in repo A
- `main` branch updated in repo B
- A comment on a PR in repo A or repo B
- PR title updated in repo A

All of a sudden you find yourself with a regexp like `^(case 1)|(case 2)|(case 3)...$` where each case has to list out all the contributing variables (with a `.*` at times).

It would be handy to be able to pass in a simple script as a filter instead.