LookupJoinExec should not assume its output but instead compute it from
- Its input fields from the left - The fields added from the lookup
index

Currently, LookupJoinExec's output is determined when the logical plan
is mapped to a physical one, and thereafter the output cannot be changed
anymore. This makes it impossible to have late materialization of fields
from the left hand side via field extractions, because we are forced to
extract *all* fields before the LookupJoinExec, otherwise we do not
achieve the prescribed output.

Avoid that by tracking only which fields the LookupJoinExec will add
from the lookup index instead of tracking the whole output (that was
only correct for the logical plan).

**Note:** While this PR is a refactoring for the current functionality,
it should unblock @craigtaverner 's ongoing work related to field
extractions and getting multiple LOOKUP JOIN queries to work correctly
without adding hacks.

(cherry picked from commit 64107e0)

# Conflicts:
#	x-pack/plugin/esql/src/main/java/org/elasticsearch/xpack/esql/planner/LocalExecutionPlanner.java