add xform on attributes in eql

[dvingo]

Fixes for EQL support with datalevin and xtdb

• plain joins can figure out the join key attribute via the fulcro component so we don't need to use -ref->attribute at runtime.
• allow support for refs that return maps as well as light entities which only contain :db/id.

Add xform support for plain attributes in addition to in recursive position.

Instead of returning ::missing-val or nil when an attribute isn't present, do not include it in the output. This is aligned with datomic pull semantics: https://docs.datomic.com/on-prem/query/pull.html#missing-attributes

In the absence of a default, attribute specifications that do not match an entity are omitted from that entity's result map, rather than e.g. appearing with a nil value.