Activate `future_snark` feature by default

[jpraynaud]

Why

Once the pre-aggregation SNARK primitives are implemented (see #2792), we can activate the future_snark feature by default. The impact should only exist in the signer and aggregator which will start creating/registering "extended" key registration including Schnorr verification key. We want to be sure that the activation of the feature has no other impact on the Mithril network and that backward compatibility is fully maintained.

What

Activate the future_snark feature by default and provide full backward compatibility for Mithril networks.

How

• Activate the future_snark feature by default in the STM library
• Check that extended verification keys are created in the signer
• Check that extended verification keys are broadcast by the leader aggregator
• Check that the backward compatibility is fully maintained for the Mithril network