Add "in-memory" or "prefer-cpu" flag for writer

[westonpace]

Generally when we write data we make decisions optimized for the case where the data is on disk. Some users have reported that they want to use Lance data for in-memory data. If we add a flag to the write options then we could allow those users to indicate this. We could also auto-enable this flag if the object storage is in-memory.

If this flag is set we could:

• Skip compression entirely
• Prefer transparent encodings
• Someday (not 2.1) we could avoid repdef

For now, using this flag to prefer transparent encodings might be the most 2.1 friendly adjustment (and should be backwards and forwards compatible)