feat: support async iterator for paging method

[xiaozhenliu-gg5]

code example:

    const client = new showcase.v1beta1.EchoClient(clientOptions);
    // ... credential and auth ....
    const resources = client.pagedExpandAsync(request);
    let counter = 0;
    for await (const resource of resources){
       counter++;
       console.log(resource.content);
       if(counter === 10) break;
    }

Node8 tests are expected to fail, because this feature is new in Node10
TODO: update interface from typescript micro-generator

[codecov]

Codecov Report

Merging #708 into master will not change coverage by %.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #708   +/-   ##
=======================================
  Coverage   88.77%   88.77%           
=======================================
  Files          43       43           
  Lines        7357     7357           
  Branches      462      462           
=======================================
  Hits         6531     6531           
  Misses        823      823           
  Partials        3        3           

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[xiaozhenliu-gg5]

Interface change in Typescript Micro-gen to make use of the async iterator is here: googleapis/gapic-generator-typescript#199
And it will be held until this PR is merged and released.

[alexander-fenster]

Thanks @xiaozhenliu-gg5! I think the functionality is now ready. All my comments are only related to style. Please fix the nits and feel free to merge the code!

As a general comment, it would be great to run prettier on the test application code since its formatting really got out of control.

[alexander-fenster]

upd. "feel free to merge the code" when we finally deprecate Node 8 and not earlier :) (we'll remove node 8 check from Kokoro first)