fix: cannot reference variables in DEFINE statement

[spena]

Description

Issue: Variables cannot be referenced in the DEFINE statement. i.e.

ksql> define env = 'qa';
ksql> define topicName = 'topic_${env}';
ksql> show variables;

 Variable Name | Value    
--------------------------
 env           | qa       
 topicName     | topic_${env}
--------------------------

This PR fixes the above issue by looking at the VariableValueContext in the VariableSubstitutor, and replace any variables referenced there.

Example:

ksql> define env = 'qa';
ksql> define topicName = 'topic_${env}';
ksql> show variables;

 Variable Name | Value    
--------------------------
 env           | qa       
 topicName     | topic_qa 
--------------------------

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 #")

[JimGalasyn]

LGTM, with a couple of suggestions.

[agavra]

LGTM!