feat: add CustomValidator

[tsugitta]

• close [RFC]: validate accepts a function as same as validateWith #6759

This adds a new validator CustomValidator, which works same as validateWith but throw ServiceValidationError associated with the field via substitutions as same as other validators.

Usage:

validate(input.value, 'Value', {
  custom: {
    with: () => {
      if (isInvalid) {
        throw new Error('Value is invalid')
      }
    },
    message: 'Please specify a different value'
  }
})

[cannikin]

Thank you!