refactor(verify-era-proof-attestation): modularize and restructure proof verification logic

- Split `verify-era-proof-attestation` into modular subcomponents for maintainability.
- Moved client, proof handling, and core types into dedicated modules.

bin/verify-era-proof-attestation/src/proof/mod.rs

@@ -0,0 +1,9 @@
+// SPDX-License-Identifier: Apache-2.0
+// Copyright (c) 2023-2025 Matter Labs
+
+mod fetcher;
+mod parsing;
+mod types;
+
+pub use fetcher::ProofFetcher;
+pub use types::Proof;