refactor(mlablocatev2): use interfaces, add missing tests, add docs (#384)

This is a very light refactoring of the mlablocatev2 package where we do
the following things:

1. use interfaces rather than depending on other pkgs where possible

2. add a missing test to the test suite

3. write more comprehensive docs (including todo-next comments)

internal/engine/internal/mlablocate/mlablocate_test.go

@@ -11,7 +11,7 @@ import (
 	"github.com/apex/log"
 )
 
-func TestWithoutProxy(t *testing.T) {
+func TestSuccess(t *testing.T) {
 	client := NewClient(
 		http.DefaultClient,
 		log.Log,