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/src/lib.rs

@@ -0,0 +1,35 @@
+use std::fmt::{Display, Formatter};
+
+mod proto {
+    tonic::include_proto!("grpc.health.v1");
+}
+
+pub mod server;
+
+/// An enumeration of values representing gRPC service health.
+#[derive(Copy, Clone, Debug, PartialEq)]
+pub enum ServingStatus {
+    Unknown,
+    Serving,
+    NotServing,
+}
+
+impl Display for ServingStatus {
+    fn fmt(&self, f: &mut Formatter<'_>) -> std::fmt::Result {
+        match self {
+            ServingStatus::Unknown => f.write_str("Unknown"),
+            ServingStatus::Serving => f.write_str("Serving"),
+            ServingStatus::NotServing => f.write_str("NotServing"),
+        }
+    }
+}
+
+impl From<ServingStatus> for proto::health_check_response::ServingStatus {
+    fn from(s: ServingStatus) -> Self {
+        match s {
+            ServingStatus::Unknown => proto::health_check_response::ServingStatus::Unknown,
+            ServingStatus::Serving => proto::health_check_response::ServingStatus::Serving,
+            ServingStatus::NotServing => proto::health_check_response::ServingStatus::NotServing,
+        }
+    }
+}