[feature] Add S3 key prefix (#4200)

Been running these changes on on my live instance without any issues as far as I can tell. It's been playing nice with multiple instances in the same bucket.

# Description

This lets users prefix their object storage files.
Useful for when you want to host multiple GTS instances inside
the same bucket. Providers like Backblaze limit the number of buckets
you can have on your account so grouping by prefix may be more desirable
in this situation.

closes #1371

## Checklist

Please put an x inside each checkbox to indicate that you've read and followed it: `[ ]` -> `[x]`

If this is a documentation change, only the first checkbox must be filled (you can delete the others if you want).

- [x] I/we have read the [GoToSocial contribution guidelines](https://codeberg.org/superseriousbusiness/gotosocial/src/branch/main/CONTRIBUTING.md).
- [x] I/we have discussed the proposed changes already, either in an issue on the repository, or in the Matrix chat.
- [x] I/we have not leveraged AI to create the proposed changes.
- [x] I/we have performed a self-review of added code.
- [x] I/we have written code that is legible and maintainable by others.
- [x] I/we have commented the added code, particularly in hard-to-understand areas.
- [x] I/we have made any necessary changes to documentation.
- [ ] I/we have added tests that cover new code.
- [ ] I/we have run tests and they pass locally with the changes.
- [x] I/we have run `go fmt ./...` and `golangci-lint run`.

Reviewed-on: https://codeberg.org/superseriousbusiness/gotosocial/pulls/4200
Co-authored-by: vdyotte <vdyotte@gmail.com>
Co-committed-by: vdyotte <vdyotte@gmail.com>

test/envparsing.sh

@@ -187,6 +187,7 @@ EXPECT=$(cat << "EOF"
     "storage-s3-bucket": "gts",
     "storage-s3-bucket-lookup": "auto",
     "storage-s3-endpoint": "localhost:9000",
+    "storage-s3-key-prefix": "",
     "storage-s3-proxy": true,
     "storage-s3-redirect-url": "",
     "storage-s3-secret-key": "miniostorage",
@@ -208,7 +209,7 @@ EXPECT=$(cat << "EOF"
 EOF
 )
 
-# Set all the environment variables to 
+# Set all the environment variables to
 # ensure that these are parsed without panic
 OUTPUT=$(GTS_LOG_LEVEL='info' \
 GTS_LOG_TIMESTAMP_FORMAT="banana" \