EncoderSimulcastProxy: Respect "supports_simulcast" info.

Encoders that do not support simulcast in the first place does not
expect to have to handle simulcast configurations, and as such may not
necessarily return
WEBRTC_VIDEO_CODEC_ERR_SIMULCAST_PARAMETERS_NOT_SUPPORTED from
InitEncode().

This CL updates EncoderSimulcastProxy to respect this info to avoid
silent errors when LibvpxVp9Encoder (which does not support simulcast)
is attempted to be used in simulcast.

Alternatively we can try to get rid of EncoderSimulcastProxy altogether
since SimulcastEncoderAdapter already has a passthrough mode. A TODO is
added to get rid of the proxy.

Bug: webrtc:14884
Change-Id: Id3703f1768b0aebf617b7d9b935914cd5f1b0f52
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/296885
Reviewed-by: Ilya Nikolaevskiy <ilnik@webrtc.org>
Commit-Queue: Henrik Boström <hbos@webrtc.org>
Reviewed-by: Erik Språng <sprang@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#39541}