Add query handlers for GRANT, SHOW GRANTS, EXISTS variants, and SHOW CREATE variants

[kyleconroy]

This commit adds parser and EXPLAIN output support for:

• GrantQuery: GRANT and REVOKE statements
• ShowGrantsQuery: SHOW GRANTS statement
• ExistsDatabaseQuery: EXISTS DATABASE statement
• ExistsDictionaryQuery: EXISTS DICTIONARY statement
• ExistsViewQuery: EXISTS VIEW statement
• ShowCreateDictionaryQuery: SHOW CREATE DICTIONARY statement
• ShowCreateViewQuery: SHOW CREATE VIEW statement

Changes:

• Add ExistsType enum to differentiate EXISTS query types
• Add GrantQuery and ShowGrantsQuery AST types
• Add ShowCreateDictionary and ShowCreateView ShowTypes
• Update parser to recognize DICTIONARY, VIEW keywords in EXISTS statements
• Update parser to handle GRANT, REVOKE, SHOW GRANTS statements
• Update parser to handle SHOW CREATE DICTIONARY/VIEW statements
• Update EXPLAIN handlers to output correct query type names
• Remove passing tests from explain_todo in metadata files