Add @MockOnly Junit 5 extension

[Stypox]

• I carefully read the contribution guidelines and agree to them.

The @MockOnly rule was introduced by #517 and then removed when migrating to Junit 5 in #768 because it was not used. Now I would need it again for #788, so this PR reintroduces it as a Junit 5 extension.

In #517 the rule also had a reason(), to make sure devs give a reason for making a test mock only, but I don't seem to be able to have Android Studio / Gradle print out ignore reasons for me, so I skipped adding that.

This PR also makes DownloaderFactory's methods static, so that there is no need to create a new instance each time and also because I needed to extract getDownloaderType from the DownloaderFactory.getDownloader method.

@XiangRongLin

[XiangRongLin]

I haven't tested it, but looks good.