[8pt] Add option to auto get unit rollup metrics file from S3

[RobHanna-NOAA]

The new run_unit_benchmark_tests.py has an optional feature to copy a pre-existing unit level metrics rollup file into the location where it can be updated as part of the unit run. It already can accept either a local file path or s3 path.

However.. We want it to be continually updated, if applicable.

The new feature? Optionally, get a new input to an s3 path to the evaluations folder. I can look for it and pull it down, instead of the user having to provide the s3 path fully to the metrics file to the latest version. With the new root s3 (adjustable) path to evals: ie) s3://ras2fim/gval/evaluations, we can check to see if it already exists and pull it down. This would likely be the most commonly used feature to keep the unit metrics file updated.

Note: This is not for the unit and version metrics file (aka.. not the one in ....PROD/12040101_102739_ble/239022 (230922 is the unit version).