Add the support for `STDDEV_POP/SAMP, VAR_POP/DEV` for star tree

We recently added the `STDDEV_POP/SAMP, VAR_POP/DEV` aggregation function. It would be great if we can support this in the star tree index.

We need to implement `ValueAggregator` interface for stddev and var.

Start by locating the star tree implementation, the ValueAggregator interface, and existing standard-deviation or variance aggregation support. Compare current aggregators with the required STDDEV_POP, STDDEV_SAMP, VAR_POP, and VAR_DEV functions. Done means the functions are supported by the star tree index with corresponding verification in the relevant tests.

Written by the indexing model from the issue text.