basic/strv: add optimizable version of strv_push/consume/extend

basic/strv: add optimizable version of strv_push/consume/extend

This will be helpful in cases where we are repeatedly adding entries
to a long strv and want to skip the iteration over old entries leading
to quadratic behaviour.

Note that we don't want to calculate the length if not necessary, so
the calculation is delayed until after we've checked that value is not
NULL.