Incremental Changelog Scan

[chinmay-bhat]

Draft PR. Waiting on incremental append scan #533 to merge.

• Integration test (test output has both Insert and Delete operations)

I created a new class ChangelogScanTask(ScanTask) to store change_ordinal and other parameters.
The plan_files() in the base class BaseIncrementalScan returns a Iterable[FileScanTask].
pyarrow.project_table() also expects tasks as Iterable[FileScanTask].

TODO:

• in BaseIncrementalScan, change return type from Iterable[FileScanTask] to Iterable[ScanTask] to allow different scan tasks.
• [needs discussion] update pyarrow.project_table() to be @singledispatch to allow different behaviours of project_table() depending on the ScanTask type.

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