Support VAR_SAMP and VAR_POP function.

[penghuo]

Description

1. Support VAR_SAMP, VAR_POP, VARIANCE, STD, STDDEV, STDDEV_SAMP and STDDEV_POP function in SQL. https://github.com/penghuo/os-sql/blob/issue99-var/docs/user/dql/aggregations.rst#var-pop
2. Support VAR_SAMP, VAR_POP, STDDEV_SAMP and STDDEV_POP function in PPL. https://github.com/penghuo/os-sql/blob/issue99-var/docs/user/ppl/cmd/stats.rst#var

Issues Resolved

#99

Check List

• New functionality includes testing.
  • All tests pass, including unit test, integration test and doctest
• New functionality has been documented.
  • New functionality has javadoc added
  • New functionality has user manual doc added
• Commits are signed per the DCO using --signoff

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