Add support for ValidatorBuilder joining/flatMapping

[alexshavlovsky]

Given:

        @Getter
        @Setter
        class Payload {
            private String field1;
            private String field2;
            private String field3;
        }

        ValidatorBuilder<Payload> vb1 = ValidatorBuilder.<Payload>of()
                ._string(Payload::getField1, "field1", c -> c.notNull());

        ValidatorBuilder<Payload> vb2 = ValidatorBuilder.<Payload>of()
                ._string(Payload::getField1, "field2", c -> c.notNull());

why not allow doing this:

        ValidatorBuilder<Payload> joinedValidator = ValidatorBuilder.<Payload>join(vb1, vb2);

and this:

        ValidatorBuilder<Payload> vb3 = ValidatorBuilder.<Payload>of()
                ._string(Payload::getField3, "field3", c -> c.notNull())
                .flatMap(vb1)
                .flatMap(vb2);

[making]

Hi, thanks for your requests.
Can you please create separate issues for each request?

[alexshavlovsky]

Actually, I think this is related to #253