Add plugins API

[arkivanov]

Add plugins API for Reaktive sources wrapping.

Closes #574

[arkivanov]

@CherryPerry @mk-jones @saket Here is a draft of plugins API. Could you please review? When this will be finalised for Observable I will expand it for other source types. Also added usage example to the MainActivity. Thanks!

[CherryPerry]

Sorry, I do not have any experience with RxJava plugins, so I can't review it from API client perspective.

[arkivanov]

@CherryPerry Maybe you could just add your general thoughts about the API. The idea is to provide an ability to wrap all created sources with custom middlewares.

[saket]

LGTM. Wanna add some documentation to ObservablePlugins.onAssemble?

[arkivanov]

Native tests are crashing, I will fix

[arkivanov]

@CherryPerry I added two commits:

1. Converted plugins API to "top level style", removed object ReaktivePlugins and replaced with top level properties and functions. This is to comply with the existing API, like reaktiveUncaughtErrorHandler, resetReaktiveUncaughtErrorHandler(), overrideSchedulers(...), etc.
2. Fixed the native SIGSEGV crash. This was due to the "undefined" property initialisation order in Kotlin, when in properties are in different files. Looks like (not only?) SharedImmutable properties are initialised at startup, and our various top-level val xxxOfEmpty properties were calling onAssembleXxx, which in turn were calling the unitialised (yet) top-level property plugins.