Implement Relaton XML / YAML schema versioning

[ronaldtse]

As per Relaton meeting 2022-10-09.

A relaton file (XML/YAML) contains two portions:

• generic Relaton content
• flavor-specific Relaton content

This task is to apply schema versioning to both parts.

The generic Relaton content version is (now) dictated by the version number here:

• https://github.com/relaton/relaton-models/releases

In YAML, it can be realized as:

---
relaton-schema-version: 1.0

In XML, it can be realized as:

<relaton ... schema-version="1.0">

The flavor-specific Relaton content version depends on the flavor schema, which needs to split out for individual versioning, is dependent on this ticket:

• https://github.com/relaton/relaton-models/issues/43

We need to first split the schemas into repos, then version those schemas independently. The version number will then apply only to that potion of content.

[opoudjis]

This means adding optional "schema-version" attributes to both bibitem and bibitem/ext.

[opoudjis]

The contents of schema-version come from the version.json file which is now generated. https://github.com/relaton/relaton-models/issues/47

@andrew2net Once you populate those values into the published gems, you can close this ticket.