Schema validation error with Postgres

[FX-HAO]

I'm running schema validation with the following command:

data-validation validate schema --source-conn source --target-conn target -tbls public.users

but got the error

Traceback (most recent call last):
  File "/usr/local/Cellar/[email protected]/3.9.5/Frameworks/Python.framework/Versions/3.9/lib/python3.9/runpy.py", line 197, in _run_module_as_main
    return _run_code(code, main_globals, None,
  File "/usr/local/Cellar/[email protected]/3.9.5/Frameworks/Python.framework/Versions/3.9/lib/python3.9/runpy.py", line 87, in _run_code
    exec(code, run_globals)
  File "/Users/fuxin.haocrypto.com/github/professional-services-data-validator/data_validation/__main__.py", line 493, in <module>
    main()
  File "/Users/fuxin.haocrypto.com/github/professional-services-data-validator/data_validation/__main__.py", line 483, in main
    validate(args)
  File "/Users/fuxin.haocrypto.com/github/professional-services-data-validator/data_validation/__main__.py", line 461, in validate
    run(args)
  File "/Users/fuxin.haocrypto.com/github/professional-services-data-validator/data_validation/__main__.py", line 421, in run
    run_validations(args, config_managers)
  File "/Users/fuxin.haocrypto.com/github/professional-services-data-validator/data_validation/__main__.py", line 400, in run_validations
    run_validation(config_manager, verbose=args.verbose)
  File "/Users/fuxin.haocrypto.com/github/professional-services-data-validator/data_validation/__main__.py", line 389, in run_validation
    validator.execute()
  File "/Users/fuxin.haocrypto.com/github/professional-services-data-validator/data_validation/data_validation.py", line 94, in execute
    result_df = self.schema_validator.execute()
  File "/Users/fuxin.haocrypto.com/github/professional-services-data-validator/data_validation/schema_validation.py", line 36, in execute
    ibis_source_schema = self.config_manager.source_client.get_schema(
AttributeError: 'PostgreSQLClient' object has no attribute 'get_schema' 

Seems we need to have a custom class extending PostgreSQLClient and implementing get_schema just like MSSQLClient or other third-party clients.
Or we can have a method just like get_ibis_table to wrap the source_client.get_schema() function and handle the difference.

I'd like to make a PR to fix this :)