Add support for stats aggregate function

[dai-chen]

Is your feature request related to a problem? Please describe.
Currently stats aggregate function support is missing in both SQL and PPL.

Describe the solution you'd like
Add support for the following stats aggregate function in SQL. MySQL reference: https://dev.mysql.com/doc/refman/8.0/en/aggregate-functions.html#function_std

1.STD
2.STDDEV
3.STDDEV_POP
4.STDDEV_SAMP
5.VAR_POP
6.VAR_SAMP
7.VARIANCE

PPL also needs to support some of them. Reference: https://docs.splunk.com/Documentation/SCS/current/SearchReference/Aggregatefunctions#stdev.28.26lt.3Bvalue.26gt.3B.29

Note that these functions may be optimized by pushing down to OpenSearch stats/extended_stats aggregation API: https://docs-beta.opensearch.org/docs/opensearch/metric-agg/#stats-extended_stats-matrix_stats. This can be part of #44 so this task can focus on basic implementation without pushdown.

Describe alternatives you've considered
N/A

[penghuo]

SQL Requirements

Refer https://dev.mysql.com/doc/refman/8.0/en/aggregate-functions.html#function_std。

• STD(expr) is a synonym for the standard SQL function STDDEV_POP().
• STDDEV(expr) is a synonym for the standard SQL function STDDEV_POP().
• STDDEV_POP(expr) is the square root of VAR_POP().
• STDDEV_SAMP(expr) is the squre root of VAR_SAMP().
• VARIANCE(expr) is a synonym for the standard SQL function VAR_POP(),
• VAR_POP(expr)
• VAR_SAMP(expr)

PPL Requirements

Refer https://docs.splunk.com/Documentation/SCS/current/SearchReference/Aggregatefunctions#stdev.28.26lt.3Bvalue.26gt.3B.29

• stddev(expr), This function returns the sample standard deviation of the values in a field. Similar to STDDEV_SAMP() in SQL.
• stddevp(expr), This function returns the population standard deviation of the values in a field. Similar to STDDEV_POP() in SQL.
• var(expr), Similar to VAR_SAMP() in SQL.
• varp(expr), Similar to VAR_POP() in SQL.

[penghuo]

Task breakdown

• Refactor the AggregationParser to support parsing extended_stats aggregation. Refactor the AggregationResponse Parser to support extended_stats aggregation #107
• Support var_pop, var_samp in SQL and PPL. Support VAR_SAMP and VAR_POP function. #112
• Support stddev_pop, stddev_samp in SQL and PPL. Support VAR_SAMP and VAR_POP function. #112