Fix massive memory leak by breaking retain cycle

[t089]

Any instance of the GenericKubernetesClient is never deallocated because there is a retain cycle between the client and its jsonDecoder through the date decoding closure which references self. This causes massive memory leak, since everything the client creates, will never be freed from memory (most notably the underlying HTTPClient).

My first idea was to use [unowned self] to break the retain cycle. But this is only safe if the decoder will never outlive the client instance. Since the decoder is a package internal property we cannot guarantee it. We could make it private, but then we might as well hide the whole date formatting internals. So, this PR just makes the date formatters owned only by the decoder. This breaks the retain cycle.

Happy New Year ;)

[iabudiab]

@t089 Hey 👋 thanks for the PR and Happy New Year to you too 🎊 🎉

As you said, the internal jsonDecoder could be private, however, I'll merge this for now and let's see in what direction the internals will develop 👍