Progress callbacks on long running adapters

Some adapter operations are time consuming, and tool authors would like to provide progress estimations. A progress callback, presumably with a percentage or other measure would enable this.

Specifically, we are looking for a proposal for an optional argument to the standard `read_from_file` and `write_to_file` functions that specifies a callback function. The callback function could take a % done amount, and maybe an informational string.

The slowest adapter we have currently is the AAF adapter, so that would be a good use case to ensure this is working as intended.

Start by reviewing the standard `read_from_file` and `write_to_file` adapter entry points, then examine the AAF adapter as the stated slow-operation use case. Done would be an agreed proposal for an optional progress callback, including its progress measure and informational data.

Written by the indexing model from the issue text.