Enable filtered aggregation in PPL

[chloe-zh]

Is your feature request related to a problem? Please describe.
The query engine has supported the filtered aggregation and SQL is enabled with this feature, but it is not enabled in PPL yet.

Describe the solution you'd like
Support filtered aggregations in PPL. For example:

POST _plugins/_ppl
{
  "query": "source=accounts | stats count() filter (where age <= 30) as age1, count() filter(where age > 30) as age2
}

Describe alternatives you've considered
Syntax TBD

Additional context
NA