Add a health checker service implementation.

- adds the code-generated health service classes to the pb along with
  a README explaining how to regenerate the generated code

- adds an implementation of the Health Checker Service along with unit
  tests and an integration test

Also:
- adds a pb folder
  : in a follow-up PR, all ruby pbs + generated code will be moved to it
diff --git a/src/ruby/Rakefile b/src/ruby/Rakefile
index 02af9a8..cc7832b 100755
--- a/src/ruby/Rakefile
+++ b/src/ruby/Rakefile
@@ -20,7 +20,8 @@
   { id: :bidi, title: 'bidi tests', dir: %w(spec/generic),
     tag: 'bidi' },
   { id: :server, title: 'rpc server thread tests', dir: %w(spec/generic),
-    tag: 'server' }
+    tag: 'server' },
+  { id: :pb, title: 'protobuf service tests', dir: %w(spec/pb) }
 ]
 namespace :suite do
   SPEC_SUITES.each do |suite|
@@ -50,7 +51,8 @@
 task 'suite:idiomatic' => 'suite:wrapper'
 task 'suite:bidi' => 'suite:wrapper'
 task 'suite:server' => 'suite:wrapper'
+task 'suite:pb' => 'suite:server'
 
 desc 'Compiles the gRPC extension then runs all the tests'
-task all: ['suite:idiomatic', 'suite:bidi', 'suite:server']
+task all: ['suite:idiomatic', 'suite:bidi', 'suite:pb', 'suite:server']
 task default: :all