Make Notify client a Plugin

[joelanman]

What

Make the GOV.UK Notify client a plug-in

Why

It used to be built-in to the kit - it's widely useful for realistically prototyping journeys that include emails and/or texts

Who needs to work on this

• developers
• designers

Who needs to review this

• developers

Done when

• [x] There's a PR to make the Notify client a plugin
• [x] Notify client appears in the available plugins list
• [x] Check docs for any updates - https://prototype-kit.service.gov.uk/docs/using-notify

[ruthhammond]

This is to be tackled in two phases. Initially this ticket is to make the Notify client available in the simplest way. Once done, a second ticket should be raised to investigate making Notify easier to use with the Kit.

[joelanman]

Missing a changelog!