feat(health): Add tonic-health server impl

This commit adds a new crate `tonic-health` which implements the
[standard GRPC Health Checking][checking] protocol.

Currently there is only a server implementation, though others have
alluded in the discussion in #135 that client implementations exist
which could also be imported as necessary.

A example server has also been added - once the client work is done a
client for this should be added also.

[checking]: https://github.com/grpc/grpc/blob/master/doc/health-checking.md

Fixes #135.

tonic-health/proto/health.proto

@@ -0,0 +1,22 @@
+syntax = "proto3";
+
+package grpc.health.v1;
+
+message HealthCheckRequest {
+    string service = 1;
+}
+
+message HealthCheckResponse {
+    enum ServingStatus {
+        UNKNOWN = 0;
+        SERVING = 1;
+        NOT_SERVING = 2;
+    }
+    ServingStatus status = 1;
+}
+
+service Health {
+    rpc Check(HealthCheckRequest) returns (HealthCheckResponse);
+
+    rpc Watch(HealthCheckRequest) returns (stream HealthCheckResponse);
+}