Add support (or README note) for custom keyword validation

[ekzobrain]

We need to extend schema validation with custom keywords. We may describe them with additional vocabularies, but we also need to make an implementation for them to be validatable. How may it be implemented? It would be great to have an official support for extending validation keyword implementation. May be it is already possible, and a README paragraph would be sufficient.
For example, we need to add support for this: https://github.com/ajv-validator/ajv-keywords?tab=readme-ov-file#uniqueitemproperties

[davishmcclurg]

There is some limited support for custom keywords. It's undocumented because it's not well supported, but you can check out the tests for examples, eg:

json_schemer/test/json_schemer_test.rb

Lines 180 to 211 in 1878fe8

	def test_it_validates_correctly_custom_keywords
	options = {
	keywords: {
	'ignored' => nil,
	'even' => lambda do |data, curr_schema, _pointer|
	curr_schema.fetch('even') == data.to_i.even?
	end
	}
	}
	schema = JSONSchemer.schema({ 'even' => true }, **options)
	assert(schema.valid?(2))
	refute(schema.valid?(3))
	options = {
	keywords: {
	'two' => lambda do |data, curr_schema, _pointer|
	if curr_schema.fetch('two') == (data == 2)
	[]
	else
	['error1', 'error2']
	end
	end
	}
	}
	schema = JSONSchemer.schema({ 'two' => true }, **options)
	assert_equal([], schema.validate(2).to_a)
	errors = schema.validate(3).map { |error| error.fetch('type') }
	assert_equal(['error1', 'error2'], errors)
	refute(schema.valid?(3))
	end

We may describe them with additional vocabularies, but we also need to make an implementation for them to be validatable. How may it be implemented? It would be great to have an official support for extending validation keyword implementation.

Ideally, I'd like to support custom keywords via custom vocabularies, but I have not done any work on that. Please let me know if you're interested in contributing!

[davishmcclurg]

More info:

• docs: document custom keyword validation #165
• 2.0.0 #135 (comment)