feat: add OpenAPI schema to release artefacts

I've not tested the release assets appear as they should since release-it has not generated a release based on this config yet. It is likely that the final "test" will have to be actually allowing a release to be made.

Use release-it's release assets feature to add the build OpenAPI schema to the release assets. Formally speaking the location of the OpenAPI artefacts is not guaranteed and so we explicitly set the location of the artifacts we use before adding it to the release-it configuration. This has the added benefit of allowing us to curate the naming of assets in releases.

We'd like the generated OpenAPI schema and API client package versions to match the release version generated by release-it.

This raises a bit of a chicken-and-egg problem in that the release assets need to be in place when release-it runs but the version number won't be bumped until release-it runs.

Work around this by adding custom configuration to the openapi:schema CI job which runs just enough release-it to "pre-bump" the version number. We don't push any changes and so this change is limited in scope to the openapi:schema job but, fortunately, downstream jobs view the version in the generated schema as canonical and so this is the only place we need to perform such trickery.

Part of #24 (closed)