feat!: Headers should be `Headers`

[danielbankhead]

Headers should be Headers:

• Creates consistency throughout the client libraries; no need to check properties for different casing for generic {} objects
  • Example: The "Content-Type" to use is ambiguous here and can lead to problems that are difficult to debug:

    const headers: {
    'content-type': 'application/json',
    'Content-Type': 'text/xml',
    };

• Creates an alignment with Request#headers
• Clients or intermediates/middleware can easily append to an existing header or create it if it does exist, like so:

  
  const headers = new Headers();

// creates, if not exist, or appends to an existing header headers.append('x-goog-api-client', 'abc');

# Migration Guide

Simply pass non-`Headers` into `Headers`.

old:
```ts
const headers = {"my-headers": "value"};

new:

const headers = new Headers({"my-headers": "value"});

Background

• Fixes: #262

🦕

[generated-files-bot[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]

This, along with #600 and #621 are going to cause breaking changes in auth + google-api-nodejs-client (and any other dependents). Since we don't have that level of integration tests, I'd strongly recommend releasing an experimental version of this library manually once you've merged all the breaking changes (but without merging the actual new release) and seeing what breaks in auth and GANC. That way you can preview breaking tests and any other changes, and you can revert anything if necessary before we release a new version of gaxios. In the future though, we would ideally just have these tests connected.