[CT-789] Add support for grant macros

[jtcohen6]

Everything in Redshift is mostly the same as in PostgreSQL, so we don't require any changes to:

• materializations
• get_grant_sql
• get_revoke_sql

However, the syntax for showing grants is very different. I think something like this will work, though I'll be the first to admit that it looks & feels pretty weird:

{% macro redshift__get_show_grant_sql(relation) %}

with privileges as (

        -- valid options per https://docs.aws.amazon.com/redshift/latest/dg/r_HAS_TABLE_PRIVILEGE.html
	select 'select' as privilege
	union all
	select 'insert' as privilege
	union all
	select 'update' as privilege
	union all
	select 'delete' as privilege
	union all
	select 'references' as privilege

)

select
    u.usename as grantee,
    p.privilege
from pg_user u
cross join privileges p
where has_table_privilege(u.usename, '{{ relation }}', privilege)

{% endmacro %}

Even though that function is called has_table_privilege, it works for both tables + views. Schema-level privileges require using has_schema_privilege instead; not something we need to worry about for now.

Assuming that query is able to return columns by the same name (and we could rename them in the SQL if need be), we shouldn't need to override any other parts of the puzzle.