Is your feature request related to a problem? Please describe.

blob.Bucket implements io/fs.FS, but currently contains a minor papercut where SetIOFSCallback() must be called before it can be used as such.

bucket := memblob.OpenBucket(nil)
defer bucket.Close()
bucket.WriteAll(context.Background(), "foo", []byte("bar"), nil)

// oops, forgot to call SetIOFSCallback()
fmt.Println(fs.Glob(bucket, "*"))

Describe the solution you'd like

One way to prevent this would be to remove Open() from blob.Bucket so that it no longer implements io/fs.FS, and replace SetIOFSCallback() with a function that returns a io/fs.SubFS:

-func (*Bucket) SetIOFSCallback(...)
+func (*Bucket) FS(...) fs.SubFS

This would make it impossible to use a blob.Bucket as an io/fs.FS without configuring a context first.

Usage would look something like the following:

 bucket := memblob.OpenBucket(nil)
 defer bucket.Close()
 bucket.WriteAll(context.Background(), "foo", []byte("bar"), nil)

-bucket.SetIOFSCallback(func() (context.Context, *blob.ReaderOptions) {
+fs := bucket.FS(func() (context.Context, *blob.ReaderOptions) {
 	return context.Background(), nil
 })
 
-fmt.Println(fs.Glob(bucket, "*"))    // Now a compile error
+fmt.Println(fs.Glob(fs, "*"))

This would be a breaking change.