ADD DataCentricFLClient as a serializable syft object

[IonesioJunior]

Description

Add DataCentricFLCLient type into the serializable objects list.

Checklist

• [X] I have followed the Contribution Guidelines and Code of Conduct
• [X] I have commented my code following the OpenMined Styleguide
• [X] I have labeled this PR with the relevant Type labels
• [ ] My changes are covered by tests