feat: Dynamic batch job array size

[l0b0]

Closes #269

Challenges:

• How to rejoin a state machine after a Choice
• Slow Internet, causing timeouts talking to the AWS API and manual deployment times rising above 20 minutes.
• Not sufficiently clear readme instructions for AWS Support to replicate problems
• Asked whether I could work around BatchContainerOverrides command Reference values having to be strings by instead reading from the batch submit job payload directly.
• Asked how to write a JSONPath to return null if a number is 1 and return the number otherwise. This doesn't seem to be possible, and was corroborated by AWS Support. AWS Support instead suggested branching the state machine to one task with no array_size and another with array_size, based on the iteration_size from the content iterator.
• mypy doesn't resolve **default_arguments to their constituent types - results in a bit of duplication between check_files_checksums_single_task and check_files_checksums_array_task creations.
• Can't use an integer payload value in a batch container override command. Similar to References always converted into strings for Step Functions definition substitutions.