[FEATURE] Add distinct_count PPL command

[anasalkouz]

This is part of the stats command with specific

[salyh]

See https://github.com/opensearch-project/sql/blob/7e73f124e2dc63a69750c8034558d6870c46ccf9/docs/user/ppl/cmd/stats.rst#example-7-calculate-the-distinct-count-of-a-field

[salyh]

It seems that the distinct_count stats function is already implemented but a) with bugs and b) without tests

I fixed the bugs and add a integ test:
eliatra@2ef5890

Not sure if i missed something but that makes it working for me.

cc @YANG-DB @anasalkouz If the code looks good for you, I am happy to open a PR

[YANG-DB]

thanks @salyh
yes, please create a PR ...