[FEATURE] Add a JSON-schema for namespace files.

[dblock]

Is your feature request related to a problem?

Coming from #483 (comment) we want to validate namespace files for certain limitations that are not present in any OpenAPI schema.

• a fixed set of x- options, e.g. x-operation-group is supported but a typo like x-opperation-group should not be
• only a fixed set of enums for x-distribution-included, -excluded
• someone uses a new and valid OpenAPI keyword that the clients should handle accordingly but it got through the PR review without updating the Client Gen doc
• #paths is not allowed in category yaml files
• #components/schemas is not allowed in namespace yaml files
• descriptions being full sentences starting with a capital letter and ending with a period.

What solution would you like?

A validated JSON schema for namespace files.

What alternatives have you considered?

Roll out tests in the linter.