fix: allow reserved keywords on variables names

[spena]

Description

Issue: The DEFINE/UNDEFINE statements do not allow variables names that matches the symbolic names found in SqlBase.g4. For instance define topic = 't1'; fails because topic is a reserved keyword. However, these should not be a problem for variables names. We should allow any variable name.

The SqlBase.g4 has a rule for nonReservedKeyword which I used initially, but this is not scalable. There are several missing keywords in that rule, and not all statements allow the same keywords as non-reserved. To scale this rule, I added an extra validation to the parser error listener to check if a syntax error is caused by a keyword being used as variable name. If so, then it returns silently without throwing any error, thus allowing the define and undefine statements to continue.

i.e.

# BEFORE
ksql> define topic = 'a';
line 1:8: mismatched input 'topic' expecting {'EMIT', 'CHANGES', 'FINAL', 'ESCAPE', 'INTEGER', 'DATE', 'TIME', 'TIMESTAMP', 'INTERVAL', 'YEAR', 'MONTH', 'DAY', 'HOUR', 'MINUTE', 'SECOND', 'ZONE', 'PARTITION', 'STRUCT', 'EXPLAIN', 'ANALYZE', 'TYPE', 'TYPES', 'SHOW', 'TABLES', 'COLUMNS', 'COLUMN', 'PARTITIONS', 'FUNCTIONS', 'FUNCTION', 'ARRAY', 'MAP', 'SET', 'RESET', 'SESSION', 'KEY', 'SINK', 'SOURCE', 'PRIMARY', 'REPLACE', 'ASSERT', 'ADD', 'ALTER', 'IF', IDENTIFIER}
Statement: define topic = 'a';
Caused by: line 1:8: mismatched input 'topic' expecting {'EMIT', 'CHANGES',
	'FINAL', 'ESCAPE', 'INTEGER', 'DATE', 'TIME', 'TIMESTAMP', 'INTERVAL', 'YEAR',
	'MONTH', 'DAY', 'HOUR', 'MINUTE', 'SECOND', 'ZONE', 'PARTITION', 'STRUCT',
	'EXPLAIN', 'ANALYZE', 'TYPE', 'TYPES', 'SHOW', 'TABLES', 'COLUMNS', 'COLUMN',
	'PARTITIONS', 'FUNCTIONS', 'FUNCTION', 'ARRAY', 'MAP', 'SET', 'RESET',
	'SESSION', 'KEY', 'SINK', 'SOURCE', 'PRIMARY', 'REPLACE', 'ASSERT', 'ADD',
	'ALTER', 'IF', IDENTIFIER}
Caused by: org.antlr.v4.runtime.InputMismatchException

# AFTER 
ksql> define topic = 'a';

ksql> show variables;

 Variable Name | Value 
-----------------------
 topic         | a     
-----------------------

ksql> undefine topic;

This fix allowed me to removed the undesired keywords from the error message.

sql> define 1 = 'a';
line 1:8: mismatched input '1' expecting IDENTIFIER
Statement: define 1 = 'a';
Caused by: line 1:8: mismatched input '1' expecting IDENTIFIER
Caused by: org.antlr.v4.runtime.InputMismatchException

The validator is on SyntaxErrorValidator, which is used as an error listener for the parsers. I just copied the syntaxError implementation from DefaultKsqlParser to this class. We can add more validators in the future if we find another error that should be valid and is difficult to add as rule in SqlBase.g4. We can also add validators to format the error messages for a better user experience.

Testing done

Added unit tests
Verified manually

Reviewer checklist

• Ensure docs are updated if necessary. (eg. if a user visible feature is being added or changed).
• Ensure relevant issues are linked (description should include text like "Fixes #")

[agavra]

lgtm!