support multiple inheritance

[jrevels]

when authoring a new schema version, it is not uncommon to want to inherit the required fields of two or more unrelated (in lineage) schema versions

This should be quite feasible/straightforward in Legolas-land, as long as we:

1. force the author to specify the order in which the multiple ancestors should be applied
2. support new syntax for this in our schema version identifiers, which captures the aforementioned ordering
3. have some basic type checking to ensure that overlapping required fields aren't mutually incompatible, i.e. later-applied parents' required fields must either be entirely disjoint with earlier parents' required fields, or be valid subtypes of them

[jrevels]

Note that it's currently pretty trivial to evaluate a predicate across a given schema version's ancestry:

function has_matching_ancestor(predicate, sv::Legolas.SchemaVersion)
    predicate(sv) && return true
    p = Legolas.parent(sv)
    isnothing(p) && return false
    predicate(p) && return true
    return has_matching_ancestor(predicate, p)
end

If we implement this, stuff like this will turn into a tree walk, which is just complicated enough that it might be worth providing some utilities around

[jrevels]

support new syntax for this in our schema version identifiers, which captures the aforementioned ordering

I think this should just be [<lineage>, <lineage>, ...]. Here's a (purposefully pretty complicated) example:

a > b > [c > d > [e,  f], g > [h, i > j], k > l > m] 

In Julia, the syntax would be:

@version FooV1 > [BarV1, BazV1] begin
    ....
end

[kleinschmidt]

Need to make sure we do this in a non-breaking way, and develop good recommendations around how to use this new power for good rather than evil.