Add Postgres adapter for `quick_are_queries_identical` and `quick_are_queries_identical`

### Describe the feature
Implement a Postgres implementation of `quick_are_queries_identical` and `quick_are_queries_identical` so that Postgres projects can use these macros.

### Describe alternatives you've considered
There are multiple strategies involving hashing that can be used to assert if two tables have exactly the same contents. These include either performing row-level hashes and comparing them across both queries/relations (`EXCEPT` could be used) or even just summarizing the whole table into a single hash and comparing those.

See a silly example below that will compare two tables. Note that this is not the implemention I would add to this package. It would be missing some wrapper to hammer it into the right output format.

```sql
with query_1 as (
select 1,2,3
),
query_2 as (
select 1,2,3
)
SELECT md5(string_agg(t::text, ',' ORDER BY 1)) FROM query_1 t
EXCEPT
SELECT md5(string_agg(t::text, ',' ORDER BY 1)) FROM query_2 t;

-- will return no rows

with query_1 as (
select 1,2,3
),
query_2 as (
select 3,2,1
)
SELECT md5(string_agg(t::text, ',' ORDER BY 1)) FROM query_1 t
EXCEPT
SELECT md5(string_agg(t::text, ',' ORDER BY 1)) FROM query_2 t;

-- will return one row with the hash of the unmatched row in query_1
```

### Additional context
Yes, it is Postgres specific.

### Who will this benefit?
Postgres projects.

### Are you interested in contributing this feature?
I would be happy to give it a shot if the project maintainers can solve these for me:
- Is there a strict benchmark for how _quick_ does this need to be deemed acceptable?
- Could you let me know your expectations regarding testing and docs for the PR? Couldn't find any standard regarding this on the repo.