fix(metadata): read XML OpenAPI externalDocs and optional attributes

[Maxcastel]

Q	A
Branch?	5.0
License	MIT

A few TypeErrors when building the OpenAPI operation from XML:

• <externalDocs description="..." url="..."/> was read from the operation node, so it always failed;
• <parameter name="format" in="query"/> without description or required (allowed by the XSD);
• <requestBody/> without required.

They now use the defaults of the OpenAPI models, like the YAML extractor.