Pluralized forms of fields

[m-mohr]

Related Issue(s): #586

Proposed Changes:

1. Pluralized field names in label extension (see Pluralization of properties that are lists #586)
2. Pluralized field name (schema => schemas) in point cloud extension

I hope I found all fields that should have plural forms.

PR Checklist:

• This PR has no breaking changes.
• I have added my changes to the CHANGELOG or a CHANGELOG entry is not required.
• API only: I have run npm run generate-all to update the generated OpenAPI files.