Move StreamConfig into its own file

Move StreamConfig into its own file, to allow it to be seen from different RtcEvent subclasses (introduces in upcoming CL).

BUG=webrtc:8111

Review-Url: https://codereview.webrtc.org/3013523002
Cr-Original-Commit-Position: refs/heads/master@{#19776}
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Cr-Mirrored-Commit: 4bb3b9c6cbc0aafcd7e9c0b385bb33ceda70ee1a