Allow setting the version comment when modifying xobjects through the REST API, and support minorRevision when deleting one

When a page's xobjects are modified through the REST API, the resulting page version always has an empty version comment, and deleting an xobject always creates a major version. Only the page resource lets the caller set the version comment (<comment> in the <page> body).

As a consequence, a client that edits object properties (for example a documentation page's structured fields) cannot explain the change in the page history, and a client deleting an object cannot mark it as a minor edit.

Current behavior

• PUT /wikis/<wiki>/spaces/<space>/pages/<page>/objects/<class>/<n>/properties/<prop> (ObjectPropertyResourceImpl): doc.save("", minorRevision)
• PUT /wikis/<wiki>/spaces/<space>/pages/<page>/objects/<class>/<n> (ObjectResourceImpl#updateObject): doc.save("", minorRevision)
• POST /wikis/<wiki>/spaces/<space>/pages/<page>/objects (ObjectsResourceImpl): doc.save("", minorRevision)
• DELETE /wikis/<wiki>/spaces/<space>/pages/<page>/objects/<class>/<n> (ObjectResourceImpl#deleteObject): doc.save(), no minorRevision parameter and no comment

The minorRevision query parameter was added to the first three in XWIKI-15120.

Proposal

• Add an optional comment query parameter to the object and object property write endpoints (PUT/POST/DELETE), used as the version comment.
• Support minorRevision on the object DELETE endpoint, like on the other object endpoints.