feat: add s3 bucket prefix config option

[MagnusHJensen]

Closes #11

This PR adds the ability to add AUTUMN_S3_BUCKET_PREFIX as an environment variable, to allow for autumn to fetch the bucket based on a prefix.

This is done since S3 Buckets needs to be globally unique second to last naming rule

Please make sure to check the following tasks before opening and submitting a PR

• [x] I understand and have followed the contribution guide
• [x] I have tested my changes locally and they are working as intended
• [x] These changes do not have any notable side effects on other Revolt projects

[DezyNation]

Is this not accurate? Why admins are not looking at it?

[Rexogamer]

We are volunteers and are focusing on other areas of the app; however, we will review this at some point.