feat: Implements basic snowflake session variables via SET/UNSET

[jsibbison-square]

Implements the very basic usages of sql variables

SET var2 = 'blue';
SELECT * from table where color = $var2;                  Produces: SELECT * from table where color = 'blue';
UNSET var2;

as described in https://docs.snowflake.com/en/sql-reference/session-variables.

List of features supported and not supported by this PR:

• [x] Variables are scoped to the session (Eg. The connection, not the cursor)
• [x] Simple scalar variables: SET var1 = 1;
• [x] Unset variables: UNSET var1;
• [x] Simple SQL expression variables: SET INCREMENTAL_DATE = DATEADD( 'DAY', -7, CURRENT_DATE());
• [x] Basic use of variables in SQL using $ syntax: SELECT $var1;
• [ ] Multiple variables: SET (var1, var2) = (1, 'hello');
• [ ] Variables set via 'properties' on the connection https://docs.snowflake.com/en/sql-reference/session-variables#setting-variables-on-connection
• [ ] Using variables via the IDENTIFIER function: INSERT INTO IDENTIFIER($my_table_name) (i) VALUES (42);
• [ ] Session variable functions: https://docs.snowflake.com/en/sql-reference/session-variables#session-variable-functions

[jsibbison-square]

These comments make sense. I'll work through these changes and notify you when its ready for review again.

[jsibbison-square]

@tekumara I've made updates from all the comments I believe this is ready for review again :).

[tekumara]

Prefect thank you!