Refactor reference implementation into smaller modules

[fjtirado]

Implementation of call task, specially grpc, http and openapi ones, requires external dependencies.
It would be nice to make this task "optional" by allowing users to select the desired modules.
This means to have a core module, with do, for, swith task and one module per call that require external dependencies.
Also, this will allow easier replacement of http task (the reference implementaion is using the jaxrs client implementation provided by glassfish, another implemetation might want to use resteasy)