Add unsubscribe_link to the client when sending emails

[willp-bl]

https://trello.com/c/Sp4nata5/21-update-java-client-library-to-send-unsubscribe-link

Changes based on those in https://github.com/alphagov/notifications-python-client/pull/241

[quis]

Will need changing from unsubscribe_link and unsubscribeLink to one_click_unsubscribe_url and oneClickUnsubscribeURL to match https://github.com/alphagov/notifications-api/pull/4084

[quis]

All looks fairly sensible to me but would benefit from a review from someone on the Notify team with a bit of Java experience. I notice you have oneClickUnsubscribeLink in lots of places. For the field in the JSON and in other clients we’ve gone with oneClickUnsubscribeURL.

Other things you could add to smooth this through:

• bump the version number
  • in src/main/resources/application.properties
  • in pom.xml
• add a corresponding entry in CHANGELOG.md, for example

  Add a oneClickUnsubscribeURL parameter to sendEmail. The unsubscribe URL will be added to the headers of your email. Email clients will use it to add an unsubscribe button.