Add signal option to horizon:purge (e.g. SIGKILL)

[crishoj]

In some cases, queue workers may become stuck in a state where they don't respond to the SIGTERM signal sent by the horizon:purge command. We've had a server where swap and memory was nearly exhausted by rogue orphans.

To make cleaning up easier, I propose adding a --signal= option to horizon:purge.

The most obvious use-case is to forcefully kill rogue orphan queue workers:

php artisan horizon:purge --signal=SIGKILL

The option accepts both numeric (--signal=9) and text (--signal=SIGKILL) signal identifiers, and defaults to SIGTERM, to ensure backwards compatibility.