Add support for distinct aggregation

[dai-chen]

Is your feature request related to a problem? Please describe.
A clear and concise description of what the problem is. Ex. I'm always frustrated when [...]

Describe the solution you'd like
Add support for the following aggregations. Note that COUNT(DISTINCT) is highest priority and others are optional. MySQL reference: https://dev.mysql.com/doc/refman/8.0/en/aggregate-functions.html#function_count

1. COUNT(DISTINCT expr)
2. AVG(DISTINCT expr)
3. SUM(DISTINCT expr)
4. MIN(DISTINCT expr)
5. MAX(DISTINCT expr)

PPL may also needs to support count distinct. Reference: https://docs.splunk.com/Documentation/SCS/current/SearchReference/Aggregatefunctions#distinct_count.28.26lt.3Bvalue.26gt.3B.29_or_dc.28.26lt.3Bvalue.26gt.3B.29

Describe alternatives you've considered
N/A

[dai-chen]

There may be limitation when there are more than one COUNT(DISTINCT) and they cannot be pushed down to OpenSearch aggregation API together. As I recall, it is translated to JOIN in other database such as MS SQL Server.