Object Storage: add context param to support request cancellation

[jiefenghuang]

What would you like to be added:

type ObjectStorage interface {
	Get(ctx context, key string, off, limit int64, getters ...AttrGetter) (io.ReadCloser, error)
}

// i.e. pass the context instead of context.Background()
func (s *s3client) Get(ctx context, key string, off, limit int64, getters ...AttrGetter) (io.ReadCloser, error) {
     ....
     resp, err := s.s3.GetObjectWithContext(ctx, params, request.WithGetResponseHeader(s3RequestIDKey, &reqID))
}

Why is this needed:

we start goroutines to handle requests for object storage, need to cancel requests in case of exceptions, such as when a timeout happened.

https://github.com/juicedata/juicefs/pull/4582/files/5036cdcfe8d3ec2aa1a3eb166432d1759c6acaa7#r1549113348