Media Reference Stream/Channel Specification support

Currently media references are focussed on locating specific assets on and identifying timing metadata for them.

When OTIO is used to drive a system that will concretely pull together media data, it will likely be necessary to identify a specific stream within a file, or even a specific channel within an audio stream.

The issue names no files, tests, or entry points; begin by clarifying the proposed media-reference stream and channel specification in the OTIO API. Done would require an agreed way to identify a specific stream or audio channel within a referenced file, with the implementation scope identified.

Written by the indexing model from the issue text.