Adds a dummy grpc demo for testing purposes

[juanpedromoreno]

This PR brings a dummy grpc Greeting example.

The underlying idea would be iterating over this example, introducing the freestyle-rpc implementation within the Freestyle ecosystem.

[codecov-io]

Codecov Report

Merging #5 into master will not change coverage.
The diff coverage is 0%.

@@         Coverage Diff         @@
##           master   #5   +/-   ##
===================================
  Coverage       0%   0%           
===================================
  Files           1    5    +4     
  Lines           1   26   +25     
===================================
- Misses          1   26   +25

Impacted Files	Coverage Δ
demo/src/main/scala/package.scala	0% <ø> (ø)
demo/src/main/scala/GreetingService.scala	0% <0%> (ø)
demo/src/main/scala/GreetingClientApp.scala	0% <0%> (ø)
demo/src/main/scala/GreetingServerApp.scala	0% <0%> (ø)
demo/src/main/scala/GrpcServer.scala	0% <0%> (ø)

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