Add ability to inject context elements into GlobalScope

[frost13it]

See motivation in #985.

To work reliably, ThreadContextElements need to be contained in a context of each coroutine that uses them, until merge of #1577.

This PR implements an alternative solution proposed by @elizarov: to give libraries an ability to inject elements into the context of GlobalScope.

Note that it does not solve the issue with suspend fun main.

[elizarov]

Unfortunately, solutions based on ServiceLoader (even fast implementation of one) are known to be problematic in Android ecosystem. See the recent hacks that were introduced to make sure that Main dispatcher impl if loaded without service loader on Android. We'll have the same problem with GlobalScope.

If the problem we are solving only relevant for SLFJ4 mapped diagnostic context? If so, could be just provide MDC-specific EmptyGlobalScope in that library only?

[frost13it]

If the problem we are solving only relevant for SLFJ4 mapped diagnostic context? If so, could be just provide MDC-specific EmptyGlobalScope in that library only?

@elizarov, my use case explained in #985 (comment), so hard-coding MDC support this way won't be suitable for me.
Is there a chance that #1577 will be merged eventually or it will be possible to access current coroutine's context via a ThreadLocal?

[elizarov]

We'll get back to #1577. There are chances we can make it work.

[frost13it]

It would be great!