feat: deferred client initialization

[alexander-fenster]

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

await client.initialize();

manually before calling any client method.