Optionally provide a callback in apply_changeset

Sometimes it is useful to be able to provide a callback function when applying changes - especially if the input diff file contains many changes. The callback could do things like

• report progress of the operation to the user
• allow early cancellation if the client is impatient
• on server side, yield to indicate that the worker is doing work (did not freeze and should not get killed)

The callback could be something like this:

bool callback(geodiff_context, caller_context, progress);

... and apply_changeset would call it every now and then (not necessarily after each statement it applies).

Should be also supported in python API.

Start by tracing apply_changeset and how the Python API exposes it. Define the callback behavior around progress, cancellation, and periodic invocation, then verify that the callback is supported in both the main API and Python API without requiring a call after every statement.