feat: fetch-Compatible API

[d-goog]

Description

This will greatly improve ergonomics for customers and our downstream libraries. Notably:

• GoogleAuth/AuthClient can be a drop-in replacement for libraries requiring fetch-API compliance

Here's an example for customers wanting this functionality:

• https://www.youtube.com/watch?v=IY-z00bfnOc&t=291s (from 4:51-5:10)

Impact

Customers can now streamline their request management to:

const auth = new GoogleAuth();
const fetchWithAuth: typeof fetch = (...args) => auth.fetch(...args);
await fetchWithAuth('https://example.com');

Testing

Added tests and updated some samples to use #fetch.

Additional Information

To avoid parsing/marshaling the returned data from GoogleAuth#fetch/AuthClient#fetch to GaxiosResponse#data, set {responseType: 'stream'} for the GaxiosOptions.
This can be useful when using this API as a drop-in replacement for fetch and the receiving API expects Response#bodyUsed to be false.
We could expose a noMarshalFetchData option here for AuthClientOptions or upstream in GaxiosOptions if this becomes a widely-desired feature.

Builds on:

• feat: fetch-Compatible API gaxios#680

🦕

[generated-files-bot]

Warning: This pull request is touching the following templated files:

• README.md - README.md is managed by synthtool. However, a partials file can be used to update the README, e.g.: https://github.com/googleapis/nodejs-storage/blob/main/.readme-partials.yaml

[sofisl]

Exciting! Just so we don't confuse people, can we keep the request documentation in the README as well (you can use fetch, or request)? Also, can we create a sample for request as well?

[d-goog]

Exciting! Just so we don't confuse people, can we keep the request documentation in the README as well (you can use fetch, or request)? Also, can we create a sample for request as well?

I'll update the README to mention both are available and will be supported long-term - I think we'll want to encourage folks use the fetch method going forward rather than request given flexibility of the API

[sofisl]

Gotcha! Maybe we can keep a request sample though and mark it as not recommended? This will also help ensure we don't break the functionality since we run sample tests.