core: refactor signed data types to interfaces

[corverroos]

Problem to be solved

See design doc: https://docs.google.com/document/d/1uGoYuYWROAfWiRwVtcd4DO7DIOVLsXkddrQarsHidhg/edit#

Core workflow data types ParSignedData and AggSignedData are partially unstructured containing signatures and raw json (bytes) data. This works well to decouple data from logic, and promotes the charon middleware architecture as a “transparent” data-signing layer in the eth2 stack.

This approach has however the following drawbacks:

• The logic working with the data (like extracting, replacing, verifying signatures) is spread out throughout the different components.
• Most components have type switches that then decode and encode different types.
• Charon isn’t actually transparent wrt data, since we do encode and decode.

Proposed solution

Refactoring these types into interfaces with proper typed implementations for the different duties would provide the following benefits:

• Data wrangling logic moved out of components and colocated in one place; the interface implementation.
• Adding new duties, would mostly be adding a new implementation.
• Components focus on what they need to do, not how it is done.