feat: make it webpackable

[alexander-fenster]

BREAKING CHANGE: This change makes google-auth-library OAuth2 functionality available for using in browsers (using webpack).

To reduce the bundle size, we use SubtleCrypto everywhere where Node.js crypto module is used. Since all SubtleCrypto methods return promises, we made two breaking changes to OAuth2Client class:

• The OAuth2Client.generateCodeVerifier method has been replaced by the asynchronous async generateCodeVerifierAsync method
• The OAuth2Client.verifySignedJwtWithCerts method has been replaced by the async verifySignedJwtWithCertsAsync method

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@48db9eb). Click here to learn what that means.
The diff coverage is 79.82%.

@@            Coverage Diff            @@
##             master     #371   +/-   ##
=========================================
  Coverage          ?   93.03%           
=========================================
  Files             ?       17           
  Lines             ?     1033           
  Branches          ?      213           
=========================================
  Hits              ?      961           
  Misses            ?       72           
  Partials          ?        0

Impacted Files	Coverage Δ
src/crypto/node/crypto.ts	100% <100%> (ø)
src/webpack.ts	100% <100%> (ø)
src/transporters.ts	96.15% <100%> (ø)
src/crypto/browser/crypto.ts	29.16% <29.16%> (ø)
src/crypto/crypto.ts	85.71% <85.71%> (ø)
src/auth/oauth2client.ts	93.89% <92.06%> (ø)

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[JustinBeckwith]

I know you're on it, but I think we should make sure to include browser tests :)

[alexander-fenster]

@JustinBeckwith One more look? :)

[alexander-fenster]

Re: changelog

From what I understand, for the basic usage of auth library, there should be no changes. Two functions were removed from OAuth2Client: generateCodeVerifier and verifySignedJwtWithCerts (replaced with generateCodeVerifierAsync and verifySignedJwtWithCertsAsync).

I'm trying to prepare a webpacked version of google-api-nodejs-client with this latest state of auth library, I'll link it to this PR when it works.

[JustinBeckwith]

@alexander-fenster this is looking awesome. After you add details on the breaking changes to the description, I can approve this and we can start cranking on v3 💃

[alexander-fenster]

@JustinBeckwith I updated the PR description above.

[JustinBeckwith]

@alexander-fenster I'll let you do the honors of merging :)

[alexander-fenster]

I. Cannot. Believe. It. Is. Merged.

Thanks @JustinBeckwith :)

[dmytrobanasko]

Facing an issue after switching from webbbpackkk branch to latest release (actually, after axios -> gaxios)
@alexander-fenster, did you try it with new client ?

[alexander-fenster]

Let's discuss it there to keep the conversation in one place :) Thanks for your report!