feat: fetch-Compatible API

[d-goog]

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

• Gaxios can be a drop-in replacement for libraries requiring fetch-API compliance
• By offering a fetch-compliant API in Gaxios we can utilize it within our own libraries, such as google-auth-library, and in turn expose a fetch-compatible API for customers downstream
  • Additionally, this would allow google-auth-library to streamline request handling by removing the Transporter concept altogether.

Here's an example for customers wanting this functionality:

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

🦕

[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