Allow functions to have root fields modified

We recently broke out 50 tables and the corresponding functions into a separate schema that are shared across multiple databases. We mainly broke them out to allow that schema to have separate database migrations that are versioned differently than the main hosting database. Unfortunately this means that now Hasura prefixes all of those functions with the name of the schema, and now all of the existing GraphQL queries are broken. There is no way to change the root fields with functions like there is for tables and views. This is an inconvenience, because now I have to go and change the source code that references these GraphQL schema functions or reproduce those functions in the public schema of the database.

Provide a way to manually rename functions like tables. This issue is related to "Optional database schema prefix" #6890.