feat!: Request Revamp

[d-goog]

Description

This is a large refactor of the auth library's internals with sweeping improvements in UX, correctness, performance, security, and scalability.

Examples:

• UX: Adopts fetch API conventions
• Correctness: Allows users to add charset to content-types and removes responseType parameter from Gaxios requests
• Performance: There is far less object-copying when things can be safely reused, especially for Headers

Testing

All existing tests pass and enhanced numerous throughout the codebase.

Impact

This revamp brings number of improvements to library:

• It returns users predictable, consistent Headers, while allowing the open, flexible HeadersInit for requests.
• Bumps our dependencies to the Node 18+ versions
• Simplifies requests:
  • With native support for the RequestInit body types, users can omit certain headers, such as content-type
• Improves cross-platform compatibility through JS standards adoption (Request, Headers, URLSearchParams, etc.)/

Additional Information

While a large PR, most of the changes are internal and doesn't affect external API consumption much outside from Headers consistency and Node 18's Headers are not case-insensitive when compared in tests (which required a lot of lower-casing header names)

A follow-up PR will add fetch support.

🦕

[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