Fix security vulnerabilities

Fix security vulnerabilities

Support for embedded structs in ValidatorFor

Fix security vulnerabilities

• Upgrade go version (to 1.19)
• Ignore non-exported fields (in v8nConstraints())
• Added OptionIgnoreOasTags (for use with ValidatorFor())

• Fixed NetIsCP - should not allow trailing :
• Added CheckExchange option to StringValidEmail constraint
• Added CheckHost option to NetIsHostname constraint

Tag parsing now uses go-andiamo/splitter - which allows for quote escaping

Added IsNull and IsNotNull constraints
Also removed constraint reference from README - now maintained in wiki

Constraints that wrap other constraint(s) now verify if the wrapped is conditional (and only check it if the conditions are met)
Affected constraints are ArrayOf, ArrayConditionalConstraint and ConstraintSet

Improve ValidatorFor options - can now pass individual options to ValidatorFor and MustCompileValidatorFor functions. Also means that these functions no longer need a 2nd nil arg when not setting options. The ValidatorForOptions struct is retained to maintain backward compatibility.