Add support for client certificates

[stephanritscher]

This adds support for providing client certificates (which can be picked from system certificate store or from file system) and for accepting self-signed single server certificates without storing them in system certificate store.
This is realized via two libraries which have minimal impact on the application code and thus are easy to maintain. MemorizingTrustManager only has been forked by me due to a build issue with current gradle version (we should switch back to the original project after my pull request has been merged), InteractiveKeyManager has been developed by me inspired by MemorizingTrustManager.

[dominiczedler]

Sorry for the delay, I will look into it as soon as I have time after my bachelor thesis. Thanks for your pull request.

[patzly]

Is it possible to keep minSdk 21? Else this would be a rather drastic change, I think a few people are running Grocy Android on old tablets e. g. in the kitchen, these devices often have early Android versions.

[stephanritscher]

Hi, this is due to InteractiveKeyManager which is my own project and not too big, so I could give it a try if you are interested in integrating the PR.

[patzly]

I see, this would be cool! Maybe you could just surround the methods that don't exist on API <23 with if/else in your library? So that the client certificats feature is maybe only working on devices with API 23+ and we would be able to keep minSdk 21?

[stephanritscher]

I think I got it - only one place to change in the end.

[stephanritscher]

Any objections so far? It's working flawlessly on my side and I just noticed a minor conflict which came up in the PR - I hope to resolve it later today.

[koostamas]

@patzly and @dominiczedler Can you review this PR? I would like to see the feature implemented.