Make ConnectivityObserver class final (#375)

* Make ConnectivityObserver class final

This is `private` to the file and will never be subclassed.

Also moved the setting of `underlyingCompletionQueue` inline to where it's declared.

* CR

Sources/SwiftGRPC/Core/Channel.swift

@@ -132,7 +132,7 @@ public class Channel {
 }
 
 private extension Channel {
-  class ConnectivityObserver {
+  final class ConnectivityObserver {
     private let completionQueue: CompletionQueue
     private let underlyingChannel: UnsafeMutableRawPointer
     private let underlyingCompletionQueue: UnsafeMutableRawPointer