Support AWS instance profile S3 credentials

[grimmdude]

Changes proposed in this pull request:

Let Aws\S3\S3Client attempt to use credentials in instance profile if key and secret are left blank.

Laravel does this currently in Illuminate\Filesystem\FilesystemManager

Reviewers should focus on:

• Ensure build is successful and existing functionality is unchanged.
• If key and secret are left blank, then Aws\S3\S3Client should attempt to use credentials in instance profile.

Screenshot

Confirmed

• [ ] Frontend changes: tested on a local Flarum installation.
• [ ] Backend changes: tests are green (run composer test).

[clarkwinkelmann]

I had to manually merge the PR to fix conflicts, and since I squashed it GitHub is unable to mark the PR as merged. I'll close the PR now, the squash merge is properly merged on master.