Performance: Put multi-value warning for Lookup Join behind a feature flag.

[julian-elastic]

Description

Optimization: Put multi-value warning behind an ES|QL flag

• Performance Improvement expected: 4-5%
• Applicability: Universal/all cases (when warning is disabled). The flag allows the warning when multi-values are encountered to be turned off, so there is no warning generated.

POC ca944a4

[elasticsearchmachine]

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