include defaults in create function signature

[joshainglis]

I noticed when altering the default value of a function parameter that migra didn't pick up that there had been a change.
This was due to only using pg_get_function_identity_arguments() for the signature, which does not include defaults.

This change adds an extra column to the functions.sql query to get pg_get_function_arguments(p.oid) as function_arguments which is then used as the signature for the create function statement. We still use the identity arguments for dropping the function.