Implement database_privileges.list_direct RPC endpoint.

[Anish9901]

Fixes #3662

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[Anish9901]

@pavish Assuming that owners are considered a direct role, I have made a slight tweak to the response suggested in the issue, the tweak being that the direct list would include either the direct privilege or the default privilege for the owning role.

• Is this behavior beneficial in any way?
• Should the response include a union of privileges for both the direct role as well as owning role?

[pavish]

@Anish9901

I think it's fine to add the default privileges of the owning role to the response when acl is null.

Is this behavior beneficial in any way?

For the beta, based on the UX we have, this isn't required. We will have the owner information from get_basic which we'll use to display access. I see no harm in having it in the response though.

Should the response include a union of privileges for both the direct role as well as owning role?

This would make it more consistent but I don't think we need to do this for the beta.

[pavish]

Do we require an additional call here?

We could use pgd.datname = db_name instead of pgd.oid=msar.get_db_oid(db_name).

[mathemancer]

LGTM. Really nice work, @Anish9901 !