feat: RPC POST for function w/single unnamed param

For POST on RPC, allows:

* passing a json object without using `Prefer: params=single-object`
  The function must be defined with a single unnamed json param and
  `Content-Type: application/json` must be specified.

* uploading binary to a function
  The function must be defined with a single unnamed bytea param and
  `Content-Type: application/octet-stream` must be specified.

* uploading raw text to a function
  The function must be defined with a single unnamed text param and
  `Content-Type: text/plain` must be specified.

BREAKING CHANGE If there's a function "my_func" having a single
unnamed json param and other overloaded pairs(with any number of
params), PostgREST won't be able to resolve a POST request to
"my_func". For solving this, you can name the unnamed json param.

my_func(json) -> my_func(prm json)

test/fixtures/schema.sql

@@ -1050,7 +1050,7 @@ create function test.ret_point_overloaded(x int, y int) returns test.point_2d as
   select row(x, y)::test.point_2d;
 $$ language sql;
 
-create function test.ret_point_overloaded(json) returns json as $$
+create function test.ret_point_overloaded(x json) returns json as $$
   select $1;
 $$ language sql;
 
@@ -2251,4 +2251,28 @@ A test for partitioned tables$$;
         foreign key (id_a, name_a) references test.partitioned_a (id, name)
       );
     end if;
-end$do$;
+end$do$;
+
+create or replace function test.unnamed_json_param(json) returns json as $$
+  select $1;
+$$ language sql;
+
+create or replace function test.unnamed_text_param(text) returns text as $$
+  select $1;
+$$ language sql ;
+
+create or replace function test.unnamed_bytea_param(bytea) returns bytea as $$
+  select $1::bytea;
+$$ language sql ;
+
+create or replace function test.unnamed_int_param(int) returns int as $$
+  select $1;
+$$ language sql;
+
+create or replace function test.overloaded_unnamed_param(json) returns int as $$
+  select $1;
+$$ language sql;
+
+create or replace function test.overloaded_unnamed_param(x int, y int) returns int as $$
+  select x + y;
+$$ language sql;