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.

[codecov]

Codecov Report

Merging #548 into master will decrease coverage by 1.43%.
The diff coverage is 80.7%.

@@            Coverage Diff             @@
##           master     #548      +/-   ##
==========================================
- Coverage   83.21%   81.77%   -1.44%     
==========================================
  Files          14       14              
  Lines        3289     3353      +64     
  Branches       72       60      -12     
==========================================
+ Hits         2737     2742       +5     
- Misses        550      611      +61     
+ Partials        2        0       -2

Impacted Files	Coverage Δ
src/v1/speech_client.ts	88.23% <80.7%> (-4.87%)	⬇️
src/v1p1beta1/speech_client.ts	88.25% <80.7%> (-4.86%)	⬇️
src/helpers.ts	93.79% <0%> (-3.88%)	⬇️

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