Validate JSON_FILE_PATH argument inputs

[ameliav]

💡 Summary

Validate that the argument submitted is a file that exists and that its a JSON file with the correct format.

Motivation and context

Validating JSON_FILE_PATH syntax improves quality control of the TPT report service and validating its existence and possibly gathering other relevant data may help improve upon the report content.

Implementation notes

• Add pymock library or equivalent to create a dummy JSON file
• Use the validator library used in issue: https://github.com/cisagov/tpt-reports/issues/36

Acceptance criteria

How do we know when this work is done?

• [ ] Only validated JSON files will be accepted