feat!: Standardize baseURL to URL’s spec

[danielbankhead]

Notable Changes

• Remove long-deprecated baseUrl
• Use proper base URL logic via new URL(url, base)

This will align us with the standard way to resolve relative-to-base URLs.

Background: https://developer.mozilla.org/en-US/docs/Web/API/URL_API/Resolving_relative_references

Migration Guide

The expected resolved URL can be determined via:

new URL(opts.url, opts.baseURL);

Example

Resolving to https://example.com/v1/mango:

• old:

const gaxios = new Gaxios({baseURL: `https://example.com/v1`});
const res = await gaxios.request({url: '/mango'});

• new:

const gaxios = new Gaxios({baseURL: `https://example.com/v1/`}); // note the ending `/`
const res = await gaxios.request({url: 'mango'}); // note the lack of leading `/`

Notice how the new functionality would allow easy access to https://example.com/mango as well by providing {url: '/mango'} (with the leading /).

🦕

[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