Add and document FileSystem connection type

[nehanene15]

Currently, a user can connect to a GCS/S3/local file like so:

data-validation connections add -c pandas Raw --json '{"source_type":"Pandas","table_name":"test","file_path":"test_data.json","file_type":"json"}'
data-validation run -t Column -sc pandas -tc pandas -tbls '[{"schema_name":"","table_name":"test"}]'

And create the Pandas connection like so:

pip install fsspec gcsfs
data-validation connections add -c pandas Raw --json '{"source_type":"Pandas","table_name":"test","file_path":"gs://dataflow-datastream-testing/test_data.json","file_type":"json"}'

We should rename the 'Pandas' connection to 'FileSystem' and build it into the CLI so that users do not need to use the 'Raw' JSON. In the case of a 'FileSystem' connection type, a 'schema_name' should not be required in the tables-list.