add validation for resource objects/resource identifier objects (missing TYPE fix)

[Jenny-Messer]

I made these changes as missing TYPE attribute in the DATA node was causing a null pointer in ResourceConverter.createIdentifier. I ended up adding some extra validation 🤷‍♀

Shortlist of changes:

• DATA node must be valid primary data:

• • a single resource object, a single resource identifier object, or null, for requests that target single resources
• • an array of resource objects, an array of resource identifier objects, or an empty array ([]), for requests that target resource collections

• TYPE and ATTRIBUTES attributes now required for resource objects

• null DATA not allowed in readDocumentCollection must be []

• null DATA still fine for readDocument

• enforcing node types on attributes (e.g. META must be a containerNode, TYPE must be a valueNode)

• INCLUDED relationships must be an array of valid resource objects, or an empty array

• RELATIONSHIPS DATA objects must be valid resource identifier objects, no exception thrown for invalid but they will be skipped along with any valid INCLUDED relationships objects (not sure if I should throw an exception here?)

[jasminb]

@Jenny-Messer thanks for the PR, will go over it and provide feedback/merge.