Add recipient-validation endpoint

[jonowar]

What

Introduces the recipient-validation/single endpoint.

This can be used to validate a single email and see how likely it is that an email sent to it would result in a success.

The endpoint

3.2.1 :001 > require 'simple_spark'
3.2.1 :002 > simple_spark = SimpleSpark::Client.new
3.2.1 :003 > simple_spark.recipient_validation.validate('[email protected]')
/Users/jono/code/simple_spark/lib/simple_spark/exceptions.rb:24:in `fail_with_exception_for_status': Forbidden. 403 (Error Code: ) (SimpleSpark::Exceptions::UnprocessableEntity)

Notes

Currently getting a 403 error on it. I wonder if maybe we need to upgrade our account.

[ColinTheRobot]

when slash if this is merged, worth opening a pr on the parent