[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>

internal/storage/storage.go

@@ -78,27 +78,32 @@ type Driver struct {
 	// S3-only parameters
 	Proxy          bool
 	Bucket         string
+	KeyPrefix      string
 	PresignedCache *ttl.Cache[string, PresignedURL]
 	RedirectURL    string
 }
 
 // Get returns the byte value for key in storage.
 func (d *Driver) Get(ctx context.Context, key string) ([]byte, error) {
+	key = d.KeyPrefix + key
 	return d.Storage.ReadBytes(ctx, key)
 }
 
 // GetStream returns an io.ReadCloser for the value bytes at key in the storage.
 func (d *Driver) GetStream(ctx context.Context, key string) (io.ReadCloser, error) {
+	key = d.KeyPrefix + key
 	return d.Storage.ReadStream(ctx, key)
 }
 
 // Put writes the supplied value bytes at key in the storage
 func (d *Driver) Put(ctx context.Context, key string, value []byte) (int, error) {
+	key = d.KeyPrefix + key
 	return d.Storage.WriteBytes(ctx, key, value)
 }
 
 // PutFile moves the contents of file at path, to storage.Driver{} under given key (with content-type if supported).
 func (d *Driver) PutFile(ctx context.Context, key, filepath, contentType string) (int64, error) {
+	key = d.KeyPrefix + key
 	// Open file at path for reading.
 	file, err := os.Open(filepath)
 	if err != nil {
@@ -144,11 +149,13 @@ func (d *Driver) PutFile(ctx context.Context, key, filepath, contentType string)
 
 // Delete attempts to remove the supplied key (and corresponding value) from storage.
 func (d *Driver) Delete(ctx context.Context, key string) error {
+	key = d.KeyPrefix + key
 	return d.Storage.Remove(ctx, key)
 }
 
 // Has checks if the supplied key is in the storage.
 func (d *Driver) Has(ctx context.Context, key string) (bool, error) {
+	key = d.KeyPrefix + key
 	stat, err := d.Storage.Stat(ctx, key)
 	return (stat != nil), err
 }
@@ -156,6 +163,7 @@ func (d *Driver) Has(ctx context.Context, key string) (bool, error) {
 // WalkKeys walks the keys in the storage.
 func (d *Driver) WalkKeys(ctx context.Context, walk func(string) error) error {
 	return d.Storage.WalkKeys(ctx, storage.WalkKeysOpts{
+		Prefix: d.KeyPrefix,
 		Step: func(entry storage.Entry) error {
 			return walk(entry.Key)
 		},
@@ -164,6 +172,7 @@ func (d *Driver) WalkKeys(ctx context.Context, walk func(string) error) error {
 
 // URL will return a presigned GET object URL, but only if running on S3 storage with proxying disabled.
 func (d *Driver) URL(ctx context.Context, key string) *PresignedURL {
+	key = d.KeyPrefix + key
 	// Check whether S3 *without* proxying is enabled
 	s3, ok := d.Storage.(*s3.S3Storage)
 	if !ok || d.Proxy {
@@ -349,6 +358,7 @@ func NewS3Storage() (*Driver, error) {
 	return &Driver{
 		Proxy:          config.GetStorageS3Proxy(),
 		Bucket:         config.GetStorageS3BucketName(),
+		KeyPrefix:      config.GetStorageS3KeyPrefix(),
 		Storage:        s3,
 		PresignedCache: presignedCache,
 		RedirectURL:    redirectURL,