add support to tag isvalid

[peczenyj]

Fixes Or Enhances

This Pull Requests adds a new tag called isvalid

If the field is marked with the validator tag isvalid, the type must implement the interface Validate() error and the return must be nil to be considered valid

A possible use case is: when dealing with Enumerations, the type can support a method Validate() error to check if the value is in specific the range defined. If we use enumer it generates a IsA<Type>() bool method that can be used to verify if the enumeration is valid or not, instead force the oneof tag (that needs to be always updated when we add one new value.

I wrote a pull request to add a Validate method on enumerations here and the interface Validate() error seems pretty common.

It may clash with existing tags that people may register, this is something that I don't know how to solve.

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: 74.338% (+0.02%) from 74.318%
when pulling d05e279 on peczenyj:add-support-to-tag-valid
into 2cce309 on go-playground:master.

[nodivbyzero]

How does this isvalid differ from custom validators?
https://pkg.go.dev/github.com/go-playground/validator/v10#hdr-Custom_Validation_Functions

validator/_examples/custom-validation/main.go

Line 11 in 2cce309

String string `validate:"is-awesome"`