Report progress events from STM during long proving sequences

[jpraynaud]

Why

mithril-stm is silent. An aggregation that loads the SRS, loads or computes two sets of circuit keys and produces a certificate proof followed by a recursive proof can run for a long time with no visible progress, which makes an operator unable to tell a slow run from a stuck one.

What

Expose progress events from the long running operations, so a caller can log them or surface them as metrics.

How

• Define the event set covering the SRS load, the key load or computation, and the proving steps
• Expose the events through a channel or a callback, with a no-op default
• Emit them from aggregate_signatures_with_type (protocol/aggregate_signature/clerk.rs) and from the IVC prover (proof_system/ivc_halo2_snark/proof.rs)
• Consume them in the aggregator logs