Enable Subsearch in PPL

[jayannah]

Is your feature request related to a problem? Please describe.
Enable subsearch capability in PPL to provide better search capability.

A clear and concise description of what the problem is. Ex. I'm always frustrated when [...]
Subsearch queries enable the search performance faster and precise results.

Describe the solution you'd like
Enable subsearch in PPL.

A clear and concise description of what you want to happen.
Use the subsearch results to the main search for the faster search query performance.

Describe alternatives you've considered

A clear and concise description of any alternative solutions or features you've considered.

Additional context

Add any other context or screenshots about the feature request here.

Here is the example. Currently Opensearch supports subsearch in SQL query format, as its not available in PPL which can be used for Alert notifications and Dashboard panels.

SELECT a.f, a.l, a.a
FROM (
SELECT firstname AS f, lastname AS l, age AS a
FROM accounts
WHERE age > 30
) AS a

[kavilla]

Hello @jayannah,

Thanks for opening. @opensearch-project/admin could you redirect this to https://github.com/opensearch-project/piped-processing-language?