[Feature Request]: Birdseye restream specific ffmpeg hwaccel options

**Describe what you are trying to accomplish and why in non technical terms**
I want to be able to set ffmpeg hwaccel options for the birdseye restream. As it is now, the birdseye restream attempts to use the h264_nvenc HW encoder when the global configuration for ffmpeg.hwaccel_args is set to preset-nvidia-h264. This is problematic when using a GPU that has only hardware decoding (ie, GT 1030), resulting in the birdseye restream crashing when attempting to connect to it. Being able to override this default just for the birdseye restream would be an added convenience.

**Describe the solution you'd like**
Add a ffmpeg.hwaccel_args subset to the birdseye config section.

**Describe alternatives you've considered**
The current workaround I've used now is to set the hwaccel_args for each camera independently and not have a global setting. This defaults the birdseye restream encoding to a non-hardware accelerated set of ffmpeg options.

No files or tests are named. Start by locating the birdseye configuration and its restream ffmpeg option handling, then trace how global hwaccel_args selects the encoder. Done means birdseye can override those options and connect successfully when the GPU supports decoding but not H.264 hardware encoding.

Written by the indexing model from the issue text.