Where UUIDs are expected change the type from String

[willp-bl]

What problem does the pull request solve?

Changes the external API to ensure that UUID are passed instead of a String

Checklist

• I’ve used the pull request template
• I’ve written unit tests for these changes
• I’ve updated the documentation in
  • notifications-tech-docs repository
  • CHANGELOG.md
• I’ve bumped the version number
  • in src/main/resources/application.properties
  • in pom.xml

[quis]

Is the improvement this gives worth making a breaking change to service’s code? Do we think consumers are handling UUIDs as UUIDs in their code, or just strings?