ESQL: Introduce `=` as an alternative to AS in RENAME

[costin]

Description

RENAME <new_field>=<old_field>
// identical to
RENAME old_field AS new_field

Rename uses AS to avoid ambiguity for renames however this is inconsistent with the = pattern used by the rest of the commands.

[elasticsearchmachine]

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