ESQL: Ensure LOOKUP JOIN works with union types

[alex-spies]

Let's add tests that ensure that LOOKUP JOIN works with union types as the match field.

I think we don't use FieldAttribute#fieldName when constructing the lookup operator, which implies that union types currently don't work at all when used in the match field.

Relates #127521 and #127521

Required for #119082

[elasticsearchmachine]

Pinging @elastic/es-analytical-engine (Team:Analytics)

[alex-spies]

Closed in #129355