Is your feature request related to a problem? Please describe.
A child of #58356

Describe the solution you'd like

1. Add grammar support in sql.y so that CRDB parser understands proper keywords and tokens similar to Postgres. reference: https://www.postgresql.org/docs/current/sql-createfunction.html
2. Properly designed AST data structures, so that CREATE FUNCTION statement can be parsed into the AST node.

Jira issue: CRDB-16924

Epic CRDB-3405