SDK v3 multistream api: draft, created only to trigger a deploy to Amplify

[marcin-bazyl]

This is just a draft PR created against master branch so that the code is deployed to Amplify. These changes are meant to be delivered to sdk_v3 branch, not master, but Amplify hosting seems to be enabled only for PRs destined for master.

[aws-amplify-us-east-2[bot]]

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[marcin-bazyl]

This won't work until we deploy a version of the webrtc-media lib that contains the multistream changes