Update the validation for loading to check the required fields as well

[lakshmi2506]

To ensure field-level permissions and successful dataset load, there are several validations performed on the mapping.yml file. These validations check whether the fields and SObjects have the required permissions, and whether namespaces need to be injected, as well as handling case insensitivity for the mapping.yml. This functionality is already implemented in the function validate_and_inject_mapping.

However, there was a missing corner case where the function did not capture errors for required fields missing in the mapping.yml. This functionality has now been added, and the function is used for the preflight check.

[lakshmi2506]

Hi @jstvz @jkasturi-sf

Please review the PR

Thank you