Add conditionally required validation

[lutfidemirci]

Nodes within schema can not accept required option. This option makes the node conditionally required. Currently we do not allow a node to be conditionally required and optional in the same time.

schema = NxtSchema.schema(:contact) do
  required(:first_name, :String)
  required(:last_name, :String)
  node(:email, :String, required: ->(node) { node.up[:last_name].input == 'Superstar' })
end

schema.apply(input: {
  first_name: 'Andy',
  last_name: 'Superstar'
}).valid? # => false

schema.apply(input: {
  first_name: 'Andy',
  last_name: 'Other'
}).valid? # => true

[getand]

A few suggestions on naming. What I would like to have though is a test that proves an error is raised in case both optional and required option are given. Otherwise looks good.

[getand]

Suggested change

	raise Errors::InvalidOptions, "Can't make a node optional and required in the same time" if has_required_option && has_optional_option
	raise Errors::InvalidOptions, "Can't make a node optional and required at the same time" if has_required_option && has_optional_option

[getand]

Suggested change

	has_required_option = options.key?(:required)
	required_option_provided = options.key?(:required)

[getand]

Suggested change

	has_optional_option = options.key?(:optional)
	optional_option_provided = options.key?(:optional)

[getand]

Suggested change

	# that it's there when the option apply.
	# that it's there when the option applies.

[getand]

Also please do not forget to update the readme with an example. ❤️