[FEATURE] [PPL] Add support to reserved keywords in PPL

[mengweieric]

Is your feature request related to a problem?
Currently for PPL query, the backtick `` has to be added to surround a reserved keyword in order for query engine to understand. Request to add reserved keyword support.

What solution would you like?
Query contains reserved keys do not need backtick just like any other non-reserved keywords

Do you have any additional context?
Example:

source = sample-data | stats count() by `index` --> index is reserved key and has to be surrounded with backtick to work

// Ideally just 

source = sample-data | stats count() by index

[Yury-Fridlyand]

Related to #672, #712 and similar issues. keywordsCanBeId list should be extended in grammar files.

[anirudha]

whats constitues a reserved keyword? how do we define what is reserved

[penghuo]

backport to 2.5. #1324