Add support for various stddev_* and var_* variants in Star-tree index

[ksnijjer]

We currently have different var functions https://docs.pinot.apache.org/configuration-reference/plugin-reference/var_pop and https://docs.pinot.apache.org/configuration-reference/plugin-reference/var_samp

as well as stddev variants https://docs.pinot.apache.org/configuration-reference/plugin-reference/stddev_pop and https://docs.pinot.apache.org/configuration-reference/plugin-reference/stddev_samp

Can we add support for these in Star-tree index? cc @Jackie-Jiang