[FEATURE]Add `earliest` PPL command

[YANG-DB]

Is your feature request related to a problem?
Add earliest command for PPL spark based execution driver .

Since spark sql doesnt offer such expression a possible option would be:

• translate the expression into a where clause

  SELECT *
  FROM sample_table
  WHERE timestamp = (SELECT MIN(timestamp) FROM sample_table);
  

• register a custom expression earliestExpression

Do you have any additional context?

• [META]Add Missing PPL commands - campaign #408

[currantw]

@YANG-DB This appears to duplicate #957. Can it be removed?