observe mini-language: consider allowing '*' in a non-terminal position

#1496 introduced `"*"` to mean "any trait" into the observe mini-language; #1525 disallowed use of `"*"` in a non-terminal position, since with current meanings and semantics it's very hard to make it useful; see the discussion in the #1525 description, and the related issue #1503.

At some point, we may want to re-introduce a more useful meaning for an expression like "*:updated" - one that's more permissive of traits on the target object that don't have an `updated` trait. There's already partial machinery for this in the expression language, in the form of the `optional` argument to expression-producing functions like `trait`, `list_items`, etc.

xref: #1496, #1503, #1505, #1525