Automate OpenAPI Spec & Postman collection generation & include in releases

[fergcb]

Regenerating the OpenAPI Spec files and the Postman collection for the API is a task which is easy to forget when working on a PR for this repo. Instead, these build artifacts could be generated automatically as part of the CI pipeline.

This can be achieved by adding a step to the github-release job of the CI workflow to generate the files, and adding an assets field to the .releaserc.json to inform Semantic Release to include the artifacts in the release.

The existing artifacts currently being tracked (/src/swagger/api-spec/* and /collection.postman.json) should be deleted and added to the .gitignore. The /src/swagger/dist/ folder should also be deleted, and the gen-postman script updated, as the built specs now live in api-spec/.