[FEATURE]Add PPL eval CASE funcitonality support

[YANG-DB]

Is your feature request related to a problem?
Current CASE functionality syntax is only available in SQL query, this functionality is important for actual use cases for Observability and in addition more general cases where eval CASE add helpful functionality to the query.

What solution would you like?
The case command will be constructed using a list of predicates and an optional ELSE default value.

CASE(<predicate1>, <value1>, [<predicate2>, <value2>, ...] [ELSE] <default>)

ELSE could be optional, removing it will save an additional keyword in PPL. It is also more like a function than a clause.

An example :

source = my_index
| eval status_category =
    case(status_code >= 200 AND status_code < 300, 'Success',
         status_code >= 300 AND status_code < 400, 'Redirection'
         status_code >= 400 AND status_code < 500, 'Client Error'
         status_code >= 500, 'Server Error',
         'Unknown')
| stats count() by status_category

Do you have any additional context?

• [META]Add PPL eval CASE funcitonality support #3008