Create validator object to give more validation flexibility

[MustafaSaber]

This will help with #2478 to try improving validation and how to get around some DataClients tests

Also this run eskip.ParseFilters by default so that's a step forward.

Related to #1618

[MustafaSaber]

Thought about making Validator interface, something like

type Validator interface {
          Validate(item interface{}) error
}

but I think it's to early for this. It was to make all these validate functions to be used in other places and interface{} wasn't the best thing to use in this place in my opinion.

[szuecs]

You can also use an interface function that doesn't pass a value as func argument.
Then you can use proper OO to figure out how to implement the Validate() error interface for a given type.

[MustafaSaber]

Real change is from L263 to EOF rest are just moved from routegroup file

[MustafaSaber]

Still need to add some extra test cases to parsing filters improvement

[AlexanderYastrebov]

Here we actually allow a list of predicates and I think dataclient does the same while parsing.
This is an undocumented feature that might be surprising for users.
Lets address this in a separate PR after we check existing cluster resources.

[AlexanderYastrebov]

Same for the filters.

[AlexanderYastrebov]

👍

[MustafaSaber]

👍