Fix long column descriptions being truncated at 63 characters in pg12 (#1368)

* Lengthen child_entities name comment to make tests check it does not get truncated at 63 chars.

* Don't cast column description to sql_identifier type (maxlen 63 chars) when introspecting schema in DbStructure.hs

src/PostgREST/DbStructure.hs

@@ -437,7 +437,7 @@ allColumns tabs =
                 nc.nspname::information_schema.sql_identifier AS table_schema,
                 c.relname::information_schema.sql_identifier AS table_name,
                 a.attname::information_schema.sql_identifier AS column_name,
-                d.description::information_schema.sql_identifier AS description,
+                d.description AS description,
                 a.attnum::information_schema.cardinal_number AS ordinal_position,
                 pg_get_expr(ad.adbin, ad.adrelid)::information_schema.character_data AS column_default,
                     CASE

test/Feature/StructureSpec.hs

@@ -125,7 +125,7 @@ spec = do
                     "type": "integer"
                   },
                   "name": {
-                    "description": "child_entities name comment",
+                    "description": "child_entities name comment. Can be longer than sixty-three characters long",
                     "format": "text",
                     "type": "string"
                   },

test/fixtures/schema.sql

@@ -1226,7 +1226,7 @@ create table ranges (
 
 comment on table child_entities is 'child_entities comment';
 comment on column child_entities.id is 'child_entities id comment';
-comment on column child_entities.name is 'child_entities name comment';
+comment on column child_entities.name is 'child_entities name comment. Can be longer than sixty-three characters long';
 
 comment on table grandchild_entities is
 $$grandchild_entities summary