concurrency: add SPI for context providers

[FroMage]

In order for mp-concurrency to replace Reactive Contexts which is currently used by RESTEasy to provide context propagation, we need to be able to define context providers via an SPI. Otherwise, context providers would be a fixed list and not support contexts that are external to MP/EE.

This can be as simple as defining a ContextProvider interface which supports:

• capturing a context
• setting a context
• restoring a context (in RESTEasy setting and restoring are different operations because the context is a stack with push/pop)

Where "the context" can be an opaque type that nobody outside of the context provider cares about (a type parameter would do).

Implementations of the ContextProvider can be listed with a META-INF/services file.

[njr-11]

To the best of my knowledge, the ThreadContextProvider SPI under pull #15 covers all of the above requirements and is conducive to context implementations that are stack-based as well as those that are not. The doc additions under that pull also specify that the ServiceLoader (META-INF/services) is the way to plug in a third-party provider. Can we close this issue if you are satisfied that everything is covered there?

[FroMage]

Yes, now that #15 has been merged, it appears to support this use-case. Thanks.