Go writer: support schemas/channels outside chunks

[wkalt]

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Adds a new option, ChannelsOutsideChunks, to the go writer.

Today in chunked MCAP files, schemas and channels are inside the chunks. I would like to be able to concatenate two nonoverlapping MCAP files without either decompressing chunks or having duplicate schemas/channels in the output. This option should make it possible to do that.

[wkalt]

I think this will require a new conformance test variant.. still studying how to do that.

[defunctzombie]

Closing for lack of activity. If it becomes important we will re-visit.