feat: Add expand-flag to RelationalBone

[phorward]

Allows for specific renders to expand the values of a RelationalBone to its full dest-skeleton, and not only the specified refKeys.

This is useful to directly fetch entire data structures with just one request, and is useful for modern single-page-apps.

Example usage:

class UserPortal(Skeleton):
    portal = RelationalBone(
        descr="Portal",
        kind="portal",
        expand=True,
        readOnly=True,
        format="$(dest.identifier)",
        refKeys=["key", "identifier", "name"],
        consistency=RelationalConsistency.CascadeDeletion,
    )

This expands portal to its full skeleton, including all descriptions, logos, marketing-huiuiui-wheeee-images and texts without spamming the datastore, by extending all bones to refKeys. The userportal entity still only stores the values in refKeys.

[sveneberth]

Yeah, of course! Otherwise there wouldn't be a difference between no and invalid data!

[sveneberth]

Do we need to think about recursion in case of nested relBones? I think this correlates in some case with #143. I mean in some cases you want to expand the sub relBone, in others not.
Imagine these relational models: A -> B -> C and D -> B -> C. The cascade of the RelationalBone in A (to B, which as a relation to C) should completely expand B and C. But the RelationalBone in D should only expand B, but not C.

[phorward]

Alternative replacement provided by #1193, issued #1101.