Add Decimal support for statistical functions var and stddev

[realno]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.

A PR is open for adding variance and standard deviation functions to DataFusion (#1525). Decimal type is not supported as Postgres spec (https://www.postgresql.org/docs/9.1/functions-aggregate.html). This issue is to capture the work.

Describe the solution you'd like
var, var_samp, var_pop, stddev, stddev_samp, stddev_pop should support decimal type.

Describe alternatives you've considered
A clear and concise description of any alternative solutions or features you've considered.

Additional context
The work can be added to (#122) or create a new PR once it's merged.

[Dandandan]

This is supported now