feat: support net.http_delete with request body

[steve-chavez]

Addresses #77 (comment). Now it's possible to do:

select net.http_delete(
    url  :='http://localhost:8080/delete_w_body'
,   body := '{"key": "val"}'
);

Backwards compatibility is tested. No body will be sent if the body parameter is NULL (the default).

Notes

• I tried adding an overloaded function for net.http_delete instead of modifying the existing one, but that caused breakage since every call like:

        select net.http_delete(
            url:='http://localhost:8080/delete'
        ,   params:= '{"param-foo": "bar"}'
        ,   headers:= '{"X-Baz": "foo"}'
        );

Failed with an error related to ambiguity (two functions with the same named parameters).

[soedirgo]

Let's only merge this after a migration file is added, otherwise pg_net.sql and the upgrade scripts go out of sync

[steve-chavez]

Ok then. I've added the migration file.