SQL: STDEV function support

[astefan]

Since #49554 has been implemented, we should consider adding STDEV function support to SQL as well.

[elasticmachine]

Pinging @elastic/es-ql (:Query Languages/SQL)

[costin]

Same for VAR, essentially introduce STDDEV_SAMP and VAR_SAMP. In SQL land there's also STDDEV and VARIANCE as historical aliases though I think in case of ES they would point to the _POP variant so for now I would just keep them as is.

[erickmp07]

I'd like to try.

[bpintea]

@erickmp07 many thanks for your offer. I'll take care of this issue.