Add support for #killsignal and #killtimeout with exec:..#backchannel=1

[jbayart]

Hi,
I'm using an exec command with #backchannel=1 parameter in the context of a two-way audio doorbell. To add a bit more context, I'm following more or less the setup described in this thread blakeblackshear/frigate#11924 (comment).
When closing the go2rtc stream, my exec command receive a SIGKILL signal and terminates immediatly without the opportunity to close properly sub commands. So I would like to use killsignal parameter to customize kill signal when stream is shut down, like specified here https://github.com/AlexxIT/go2rtc?tab=readme-ov-file#source-exec. Basically I'm having the same issue than #1193 but with a command using backchannel context.

Unfortunelty, from what I've experienced with go2rtc and what I've seen in the code, it seems like these parameters are not taken into account when exec command is used with backchannel, so I think it would be a great addition.