Unary Health Check Service should support not only protobuf but also avro

[ememmr]

There's a first version of health check service resolved in issue #626. Right now, unary service uses protobuf protocol, it should also support avro.

Here, the improvement suggestion:

[...]
We could have the following modules:
* `health-check-protocol-unary`: With the current unary `@service`
* `health-check-service-unary`: Depending on `channel` and `health-check-protocol-unary-scala` with the `% provided`

We could also include the `avdl` and `proto` files in the `channel` resources, providing instructions about how to add the sbt settings for generating your protocol definition.
[...]

Originally posted by @fedefernandez in #630 (comment)