Add ALTER FUNCTION

[homar]

Trino already supports ALTER SCHEMA and ALTER TABLE, however ALTER FUNCTION is missing.

It should be possible to modify function's body and/or returned type and set function owner.

Proposed syntax for body/return value modification:

ALTER FUNCTION catalog.schema.function_name
[RETURNS BIGINT]
[BODY]

For setting authorization:

ALTER FUNCTION function_name SET AUTHORIZATION ( user | USER user | ROLE role )

[homar]

Subissue for SET AUTHORIZATION part: #25021

[electrum]

We already support changing the function definition with CREATE OR REPLACE FUNCTION, which is the same as how you replace a view definition. However, as with views, supporting ALTER ... SET AUTHORIZATION makes sense.