feat: deferred client initialization (#370)

This PR includes changes from googleapis/gapic-generator-typescript#317
that will move the asynchronous initialization and authentication from the client constructor
to an `initialize()` method. This method will be automatically called when the first RPC call
is performed.

The client library usage has not changed, there is no need to update any code.

If you want to make sure the client is authenticated _before_ the first RPC call, you can do
```js
await client.initialize();
```
manually before calling any client method.

synth.metadata

@@ -1,13 +1,13 @@
 {
-  "updateTime": "2020-02-29T12:46:32.088143Z",
+  "updateTime": "2020-03-05T23:17:35.829582Z",
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(Otherwise, it will not build.)\n\nPiperOrigin-RevId: 294264582\n\n6d9361eae2ebb3f42d8c7ce5baf4bab966fee7c0\nrefactor: Add annotations to Bigtable Admin v2.\n\nPiperOrigin-RevId: 294243406\n\nad7616f3fc8e123451c8b3a7987bc91cea9e6913\nFix: Resource type in CreateLogMetricRequest should use logging.googleapis.com.\nFix: ListLogEntries should have a method signature for convenience of calling it.\n\nPiperOrigin-RevId: 294222165\n\n63796fcbb08712676069e20a3e455c9f7aa21026\nFix: Remove extraneous resource definition for cloudkms.googleapis.com/CryptoKey.\n\nPiperOrigin-RevId: 294176658\n\ne7d8a694f4559201e6913f6610069cb08b39274e\nDepend on the latest gapic-generator and resource names plugin.\n\nThis fixes the very old an very annoying bug: https://github.com/googleapis/gapic-generator/pull/3087\n\nPiperOrigin-RevId: 293903652\n\n806b2854a966d55374ee26bb0cef4e30eda17b58\nfix: correct capitalization of Ruby namespaces in SecurityCenter V1p1beta1\n\nPiperOrigin-RevId: 293903613\n\n1b83c92462b14d67a7644e2980f723112472e03a\nPublish annotations and grpc service config for Logging API.\n\nPiperOrigin-RevId: 293893514\n\n"
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