[Feature][function_score] Execute a function on scores in score_mode

[synhershko]

Currently score_mode only supports a basic set of operations on scores provided by the functions executed in function score query: multiply, sum, avg, first, max, min.

It will be very useful to have the ability to execute a more involved function on them instead, e.g.: _score1 + _score3 - _score2*_score5, where scores can be numbered or names after the functions order / names.

[dakrone]

Related to #19710

[clintongormley]

Closing in favour of #19710

[synhershko]

Thanks @clintongormley @dakrone - as long as that solution allows for sibling scores to be passed as parameters to that script, that would work.