Review validation constraints packaging

[yuri-sergiichuk]

The io.spine.validate package is getting too big. We should consider making different constraints more pluggable for the possible extension of the validation framework.

Also, by making constraints pluggable, it will be probably possible to move them to separate packages.

[alexander-yevsyukov]

The io.spine.base package depends on io.spine.validation in a number of places, but this dependency can be avoided with some efforts.

The hardest dependency is made in the proto type Error, which declares the field validation_error:

message Error {
    ...
    // An error indicating that a message did not pass validation.
    validate.ValidationError validation_error = 3;
    ...
}

The rest of the dependencies are implications of this one.

[alexander-yevsyukov]

@armiol, in the light of introducing Validation as a separate library, do you think we need to address it in the scode of v2.0?

[armiol]

@alexander-yevsyukov yes, definitely.