Add Utilities for Interacting with Ingest Status table

[medley56]

Context

The ingest status table is written to and read from during ingest processing by a Lambda whose code is externally defined by the user. We should provide utilities for interacting with this table as part of the Construct Library so users can use standard reads/writes on the table without having to write their own DynamoDB code.

Implementation

We could use a library like PynamoDB for this, but then we would be levying that requirement on the users. It's easier if we just directly abstract the Dynamo queries into functions that the user Lambdas can use to interact with the status table.

We should also document the schema of the ingest status table as part of this issue. Write a markdown file that describes how the ingest status table should be used to track processing of files. Note that since Dynamo is schema on write, the user can pretty much do whatever they want with the table during processing.