validate schema: --allow-list option prevents some required mappings

[nj1973]

There is a secondary check in the code behind --allow-list that ensures the base data type is the same in source and target.
It will accept mapping of decimal(10,0) to decimal(8,0) but rejects decimal(10,0) to int64 (bigint). This is rejected because the base data type of decimal is decimal and that is not the same as the base datatype of int64 which is int.

This prevents these useful mappings:

• date -> timestamp
• timestamp -> date
• decimal(?,0) -> int[16/32/64]

We need to understand why these mappings were prevented and, ideally, extend the accepted base types to include those above.

[nehanene15]

Issue #706 is related - timestamp -> timestamp('utc') could not be passed into the allowlist