asset-swapper: use RFQT-specific response types

@0x/quote-server was recently updated to offer RFQT- and RFQM-specific
types, in addition to abstracted types.  Since everything here is RFQT
specific, usage has been changed to use those specific types.

Addresses review comments
https://github.com/0xProject/0x-monorepo/pull/2582#discussion_r437623138
and
https://github.com/0xProject/0x-monorepo/pull/2582#discussion_r437625305

packages/asset-swapper/test/quote_requestor_test.ts

@@ -188,7 +188,9 @@ describe('QuoteRequestor', async () => {
                         intentOnFilling: true,
                     },
                 );
-                expect(resp.sort()).to.eql([successfulOrder1, successfulOrder2].sort());
+                expect(resp.sort()).to.eql(
+                    [{ signedOrder: successfulOrder1 }, { signedOrder: successfulOrder2 }].sort(),
+                );
             });
         });
     });