[FEATURE]Add PPL Explain Command / Endpoint

[YANG-DB]

Is your feature request related to a problem?
In a similar way that OpenSearch PPL can create a query with explain flag that will produce the execution plan / actual query that is being run against the server, Spark PPL needs a matching capability

What solution would you like?
Create an explain flag attribute that will respond with the logical / execution plan for the given query.

Do you have any additional context?

• https://github.com/opensearch-project/sql/blob/main/docs/user/ppl/interfaces/endpoint.rst#explain

[kt-eliatra]

@YANG-DB

I prepared a working prototype ae687d6

If it looks good for you I'll prepare a PR

[YANG-DB]

Nice - please add that to the comments of the issue itself and we can discuss it further...
In addition I would add a | after the 'explain' command and remove the '=true ' ...