Add Bun file FileFetcher

[markhallen]

What are you trying to accomplish?

This PR adds the fetcher for bun.lock files. A later PR will add support for legacy bun.lockb files.

We are limiting fetching the bun files with the experiment bun_updates as an experimental feature.

• This PR is a follow-up to Add bun package manager #11267 and Add BunLock FileParser #11268
• Contributes to add support for bun #6528

Anything you want to highlight for special attention from reviewers?

• Is the usage of Experiments.enabled?(:bun_updates) sufficient?

For wider context you can check #11209 or #11248

The ultimate plan is to use a new structure for multiple Javascript ecosystems similar to #1862.

This PR was put together before the change in approach. It was decided that we proceed with these PR's as-is.

We will then follow-up with a PR migrating to the new structure. This migration PR will then serve as an example for the migration of the other Javascript ecosystems.

How will you know you've accomplished your goal?

The new specs should pass and the existing specs should be unimpacted.

Checklist

• I have run the complete test suite to ensure all tests and linters pass.
• I have thoroughly tested my code changes to ensure they work as expected, including adding additional tests for new functionality.
• I have written clear and descriptive commit messages.
• I have provided a detailed description of the changes in the pull request, including the problem it addresses, how it fixes the problem, and any relevant details about the implementation.
• I have ensured that the code is well-documented and easy to understand.

The base branch was changed.

[thavaahariharangit]

LGTM