gRPC integration with MicroProfile Context Propagation

[njr-11]

User Stories:
"As a user of Liberty, I want gRPC context to be propagated to dependent completion stage actions (via MicroProfile Context Propagation) so that invoke gRPC operations from any dependent stage, regardless of which thread it runs on."

"As a user of Liberty, I want the absence of gRPC context to be propagated to dependent completion stage actions (via MicroProfile Context Propagation) so that completion stage actions do not inadvertently run with and unexpected gRPC context (such as that of a thread which request the result or completes another stage in the hierarchy upon which it depends)."

The following issue represents the requirement on the MicroProfile Context Propagation specification side,
eclipse/microprofile-context-propagation#159
All of the work should be done under the gRPC feature first (will involve supplying a ThreadContextProvider implementation for gRPC either from the gRPC binary, or independently by Liberty - which would require support in gRPC for context clearing).
The ThreadContextProvider will choose a constant to indicate gRPC context, a value such as "gRPC". After this is all implemented, then the MP Context Propagation spec can be separately updated to reference the constant for the gRPC context type.