VostokMultiHost singletons

[HolyPrapor]

https://kontur.slack.com/archives/C08DRHZTP/p1613624274446400

The goal here is to make users able to register singletons created from their configurationProvider.Get<MySettings>

[HolyPrapor]

It may be not related to this issue, but I don't want to create a separate one now.

I received a feedback that it's hard to work with static members when migrating from separate processes to VostokMultiHost.

For example, it is hard to use static ConfigurationProvider because VostokMultiHost initializes it only once.
Another example is that sometimes we don't want to initialize some components twice (or thrice or n times for MultiHost and for child applications).

Needs a research...