Support `included` in create/updates

[ttrmw]

The resource parser currently ensure!s that data is the only top level key: https://github.com/jsonapi-rb/jsonapi-parser/blob/1ab3d3561f9f82a6f859cab8094fb5e4fa413255/lib/jsonapi/parser/resource.rb#L14

but the spec does not require this. There also appears to be a bunch of discussion around sideposting and the use of included to define resources for sideposting, eg https://github.com/json-api/json-api/pull/1197, so I guess this will be required eventually.

[beauby]

This has been on my radar for a while, but I'll wait until sideposting is fully spec'ed before changing the lib.