Enhance the sync functionality or add a pipeline-like feature

[umialpha]

What would you like to be added:
**Enhance the sync functionality or add a pipeline-like feature? **

the usage of juicefs sync is soooo good! It can essentially replace many cloud data synchronization tools. The performance is excellent.
Sometimes, while synchronizing data, we need to perform some processing on the data. The most common tasks are zipping/unzipping or additional processing, such as checking whether images are corrupted.

For example, I added a Pipeline interface in the config in my personal branch. I defined a interface

type Pipeline interface {
Filtered(obj object.Object) bool
Do(src, dst object.ObjectStorage, obj object.Object, config *Config) error
}

My current implementation is quite hacky, so I'd like to ask if the community is interested in this idea.

[umialpha]

Another idea is to treat sync as a type of pipeline as well, so that pipelines can be chained together.

[jiefenghuang]

Sounds good, but seems it require a plugin mechanism? @zhijian-pro

[zhijian-pro]

The plugin mechanism conflicts with static compilation and is not a good solution