FDW JSONSchema fixes

[mildbyte]

Fixes to JSONSchemas for Socrata, Snowflake and CSV in S3.

CSV and Snowflake now have a changed JSONSchema (some credentials are pushed down into a separate object for better oneOf ergonomics), so added a compatibility layer to their __init__ that lets them accept both old and new JSONSchemas. For example, the CSV plugin will accept both:

            {
                "s3_endpoint": "objectstorage:9000",
                "s3_secure": false,
                "s3_bucket": "test_csv",
                "delimiter": ","
            }

and

        {
            "delimiter": ",",
            "connection": {
                "connection_type": "s3",
                "s3_endpoint": "objectstorage:9000",
                "s3_secure": false,
                "s3_bucket": "test_csv",
            },
        }

This will let us kick down the road the problem of figuring out migrations for these configs.

Also, some minor stylistic fixes to other JSONSchemas (delete useless null type, add minimum/maximum).

CU-1gh1c5k