Validate the encoder for dual output before going live

[mhoyer-streamlabs]

A user is able to select an encoder in Output settings based on the main stream service, but if they go live with Dual Output that encoder is never validated for the additional service. If the encoder is invalid, the stream fails but the user isn't notified and no errors are logged. Added a function to the backend to validate encoders and use that to prevent this from happening. On Go Live, validate the encoder if using Dual Output but NOT using the restream server - that is handled elsewhere. If the encoder is invalid, prompt the user to default to x264 or select their own in settings.

To reproduce:

1. Go to Stream settings and select Stream to custom ingest
2. Select Streaming Services as stream type and select YouTube RTMPS
3. Click Use recommended settings
4. Go to Output settings and select the encoder NVIDIA NVENC AV1
5. Go Live and select Dual Output, choose YouTube and any other destination (I tested with Twitch and Kick)
6. Confirm and Go Live will prompt for an encoder change