User-defined Function Support

[fulghum]

Dolt currently supports creating stored procedures, but does not yet support creating user-defined functions. Stored procedures allow customers to execute custom SQL code, but not directly return results that can be used in other parts of queries.

While working with a customer on a migration, we found their schema was using multiple user-defined functions to generate values used in other views and queries. Allowing users to create user-defined functions would enable these use cases with Dolt.