decimal support for agg function

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The decimal data type has been supported in #122, but in this issue #122 we just support several agg functions.
In this issue we will list some functions which are not implemented for the decimal data type.

- [x] sum https://github.com/apache/arrow-datafusion/pull/1408

- [x] min/max https://github.com/apache/arrow-datafusion/pull/1407

- [x] count/DistinctCount: don't need to implement

- [x] avg https://github.com/apache/arrow-datafusion/pull/1408

- [ ] var(col1): calculate the variance (sample) of col1

- [ ] var_samp(col1): calculate the variance (sample) of col1

- [ ] var_pop(col1): calculate the variance (population) of col1

- [ ] stddev(col1): calculate the standard deviation (sample) of col1

- [ ] stddev_samp(col1): calculate the standard deviation (sample) of col1

- [ ] stddev_pop(col1): calculate the standard deviation (population) of col1

TODO

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Start by reading the decimal support from #122 and the existing aggregate work in PRs #1407 and #1408, then trace how sum, min/max, and avg handle decimal values. Implement decimal support for var, var_samp, var_pop, stddev, stddev_samp, and stddev_pop, and verify that each function returns the correct sample or population result.

Written by the indexing model from the issue text.