feat: Validate fields according to Optional, Required, Mandatory

With error flags / wrapping we should validate fields according to these requirements instead of what we currently have. Often we have to relax validations (see [#1150](https://github.com/moov-io/ach/pull/1150) as an example) when our validations aren't aware of Required fields.

Related with https://github.com/moov-io/ach/issues/1050