fix(metadata): accept openapi and priority on XML parameters

[Maxcastel]

Q	A
Branch?	5.0
License	MIT

In XML the extractor reads <openapi> and priority on a <parameter>, but the XSD only allowed an <openApi> child (typed as an OpenAPI operation, and never read) and no priority attribute. So this was rejected:

<parameter key="q" priority="10">
    <openapi name="q" description="Search"/>
</parameter>

The XSD now accepts both (<openApi> is still allowed so existing files don't break), and missing attributes use the defaults of the OpenAPI model instead of null.