Export encoding+reason in RemoteTrack

[sgfn]

Description

Add encoding and encodingReason fields to RemoteTrack

Motivation and Context

Display currently active layer
FCE-2939

Documentation impact

• Documentation update required
• Documentation updated in another PR
• No documentation update required

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to
  not work as expected)

[Karolk99]

We have e2e tests in this repo. I'm not sure if they test switching variants, but if they do, it's a good idea to check if the encoding value changes.

[sgfn]

We have e2e tests in this repo. I'm not sure if they test switching variants, but if they do, it's a good idea to check if the encoding value changes.

The react-client e2e tests don't cover encoding/variants at all. The webrtc-client e2e tests exercise setTargetTrackEncoding at the lower layer, but nothing touches usePeers. I'd leave it as-is