fix: HEAD unnecessarily executing aggregates

[steve-chavez]

Previously:

curl -I localhost:3000/projects -H "Prefer: count=exact"

tail -f /run/user/1000/postgrest/postgrest-with-postgresql-15-vGF/db.log

WITH pgrst_source AS ( SELECT "test"."projects".* FROM "test"."projects"     )  SELECT null::bigint AS total_result_set, pg_catalog.count(_postgrest_t) AS page_total,
coalesce(json_agg(_postgrest_t), '[]') AS body, -- note here
nullif(current_setting('response.headers', true), '') AS response_headers, nullif(current_setting('response.status', true), '') AS response_status FROM ( SELECT * FROM pgrst_source ) _postgrest_t

Now:

curl -I localhost:3000/projects -H "Prefer: count=exact"

tail -f /run/user/1000/postgrest/postgrest-with-postgresql-15-N94/db.log

WITH pgrst_source AS ( SELECT "test"."projects".* FROM "test"."projects"     ) , pgrst_source_count AS (SELECT 1 FROM "test"."projects") SELECT (SELECT pg_catalog.count(*) FROM pgrst_source_count) AS total_result_set, pg_catalog.count(_postgrest_t) AS page_total, 
''::text AS body, -- note here
nullif(current_setting('response.headers', true), '') AS response_headers, nullif(current_setting('response.status', true), '') AS response_status FROM ( SELECT * FROM pgrst_source ) _postgrest_t

It's a bit hard to test it as mentioned on #2849 (comment) but the new types make sure this behavior is clear. I've added a ResultAggregate(previously on #2825) that has a NoAgg value.

Closes #2849.