Feature/new event type

[luke-hill]

Description

Changes to use a new event type structure (Which should only be a transitory one as we move away to just envelope events - then no events).

Type of change

Please delete options that are not relevant.

• Refactoring (improvements to code design or tooling that don't change behaviour)

Please add an entry to the relevant section of CHANGELOG.md as part of this pull request.

Note to other contributors

If your change may impact future contributors, explain it here, and remember to update README.md and CONTRIBUTING.md accordingly.

Checklist:

Your PR is ready for review once the following checklist is
complete. You can also add some checks if you want to.

• Tests have been added for any changes to behaviour of the code
• New and existing tests are passing locally and on CI
• bundle exec rubocop reports no offenses
• CHANGELOG.md has been updated