ESQL: Patch kibana signature for BUCKET (elastic#133682)

Our signature handling code considers the second and third arguments of
BUCKET to be optional - the railroad diagram and function registry
consider those arguments optional.

But in another sense, these function arguments aren't optional - they
are required IF you are invoking the certain types.

This patches the generated kibana signature to it'll look and feel
right.

Author: nik9000