refactor(stunreachability): input required and must be an URL (#630)

Here we're refactoring stunreachability to not provide internally a
default input and to take in input an URL rather than a string.

The related ooni/spec change is https://github.com/ooni/spec/pull/227.

This diff has been extracted from https://github.com/ooni/probe-cli/pull/539.

Because the original diff was large, I'm splitting it in a set of
more easily manageable diffs.

The reference issue is https://github.com/ooni/probe/issues/1814, which
is complex enough to require us to proceed incrementally.

This diff WILL need to be backported to release/3.11.

internal/engine/experiment/stunreachability/fake_test.go

@@ -6,6 +6,9 @@ import (
 	"time"
 )
 
+// TODO(bassosimone): we should use internal/netxlite/mocks rather
+// than rolling out a custom type private to this package.
+
 type FakeConn struct {
 	ReadError             error
 	ReadData              []byte