feat: Add support for omitting empty and zero values in validation (including nil pointer and empty content of pointer)

[zeewell]

Fixes Or Enhances

Add the omitzero Tag:

• It allows you to ignore the subsequent validations if the value of a field is empty
• If the field is a pointer and the pointer is nil, ignore the subsequent validations
• If the field is a pointer and the pointer is not nil, but the content value of the pointer is zero, the subsequent validations will be ignored

Make sure that you've checked the boxes below before you submit PR:

• Tests exist or have been written that cover this particular change.

@go-playground/validator-maintainers

[coveralls]

coverage: 74.319% (+0.03%) from 74.291%
when pulling 835464d on zeewell:feature/omitzero
into a947377 on go-playground:master.

[zeewell]

Fixes #249 #485 #698 #849