[Core] Replace pickle with language-independent serdes

[mosharaf]

Description

Throughout the code, FedScale pickles data before shipping them, which assumes python on both ends. Ideally everywhere, but at the very least, between components, we should avoid pickle so that components written in two different language/runtime can interact.

Use case

Our workers can be written in Python, but also in Kotlin or Swift if the clients are on mobile phone.