[POST-MVP] Add API endpoint to validate one row

[aharjati]

Add API endpoint that receive and validate one row data.

This endpoint shall receive data in JSON format with keys sets to FIG column names

This endpoint shall return response in JSON format.
When the data contains error(s), response shall contain error code(s) and message(s)
When the data is valid, response shall contain successful message.

[aharjati]

@hkeeler this is ticket to track the validate endpoint API work. do you have any details to add/change?

[hkeeler]

Here's the branch with structure output for validation results.

https://github.com/cfpb/regtech-data-validator/compare/structure-output

Most of the interesting bits related to how we may want to structure our API response is all in main.py.

This also includes changes to is_unique_column. Something about the Series being returned was making the JSON serialization fail. It's working now.

And there's also quite a few type checker fixups in there.