RJS-2732: Support updating the `App`'s base URL via experimental export

[elle-j]

What, How & Why?

Experimental exports/imports

• An experimental directory has been added (packages/realm/src/experimental), intended for exporting various experimental features.

Base URL

• Importing "realm/experimental/base-url" will extend the App with the instance members App.baseUrl and App.updateBaseUrl(), allowing the user to retrieve and update the base URL.

TSConfig / Choosing module resolution

• Importing the experimental package will only work with certain resolution algorithms. Example tsconfig.json:

{
  "compilerOptions": {
    "target": "es2022",
    "module": "node16",
    "moduleResolution": "node16",
    // ...
  }
}

Example Usage

import { App } from "realm";
import "realm/experimental/base-url";

async function exampleFunction() {
  const app = new App("my-atlas-app-id");

  // Get the currently used base URL.
  const currentBaseUrl = app.baseUrl;

  // Update the base URL.
  await app.updateBaseUrl("https://my-new-url.com");

  // Reset it to the default one.
  await app.updateBaseUrl(null);
}

This closes #6486

☑️ ToDos

• [x] 📝 Changelog entry
• [x] 🚦 Tests (but need to add follow-ups to skipped unimplemented tests)

[elle-j]

Just reviving the old draft implementation then I'll be modifying the test and adding some more 🙂

[elle-j]

@kneth, for the final review there are only minor changes to look at:

• CHANGELOG.md
• packages/realm/src/experimental/base-url.ts: Per another discussion, the API updateBaseUrl() now accepts null as a way to reset the base URL to the default one (in addition to empty string).

Still missing:

• Haven't got the mock fetch to work yet, so I'll add a ticket to implement the skiped tests.