PoC implementation with protobuf

[jpraynaud]

PoC Scenario

Add/Remove/Update a field message_type on the single signature that is sent from the Signer to the Aggregator:
In that case, the Aggregator would receive 2 different kinds of message from the updated/non-updated signers.
We want to avoid breaking changes that would prevent the network from producing certificates for one epoch.

To do

• Assess technical constraints of the protobuf implementation.
• Develop prototypical implementations of protobuf implementations

⚠️ the minimum requirement for the implementation is that the end to end test is working

Parent issue

#673

[ghubertpalo]

A PoC has been set up in a separate project. The project required the installation of protoc the Protobuf schema compiler. Once installed, the compiler was able to compile a valid schema file to a Rust implementation. It did not create any migration structures between 2 versions nor even propose a versioning scheme. The fallback happens due to all fields having a default value set.
That would require keep a version information in the Protobuf schema file and share it between nodes.

The exact same behavior is already set up with openapi.yml file and serde default values.