feat: introduce signer abstraction for secret key operations

[Gabriel-Trintinalia]

PR Description

This pull request is a non-functional refactor that introduces an abstraction for private-key cryptographic operations as part of Besu’s Discovery v5 implementation. No protocol logic or runtime behaviour is changed.

Motivation

The goal is to allow consumers of the discovery library to handle private key operations without exposing the node’s private key to discovery components. Instead of passing a SecretKey throughout the codebase, discovery now depends on a narrow interface that performs only the required cryptographic actions.

Summary of Changes

• Introduced the Signer interface to abstract signing, ECDH key agreement, and public key derivation.
• Added DefaultSigner, an in-memory Signer implementation backed by a SECP256K1 SecretKey, providing identical behaviour to the previous direct usage.
• Replaced all direct SecretKey usage with Signer across discovery initialisation, session management, handshake logic, and node record handling.
• Updated builders and identity schemas to route all cryptographic operations through the new abstraction.
• All cryptographic behaviour and Discovery v5 semantics remain unchanged.

---

Note

Introduces a private-key operation abstraction without altering Discovery v5 behavior.

• Add crypto.Signer interface and in-memory DefaultSigner backing SECP256K1
• Replace SecretKey with Signer in DiscoverySystemBuilder, DiscoveryManagerImpl, NodeSession/NodeSessionManager, and pipeline handlers (WhoAreYou*, Handshake*)
• Route ENR signing/updates via Signer: updates to IdentitySchemaInterpreter/V4, NodeRecord, NodeRecordBuilder, LocalNodeRecordStore
• Extend Functions.hkdfExpand to accept Signer for ECDH; update handshake key derivation and ID-signing (HandshakeMessagePacket)
• Update tests and helpers to use DefaultSigner

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[github-actions]

All contributors have signed the CLA ✍️ ✅
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[Gabriel-Trintinalia]

I have read the CLA Document and I hereby sign the CLA

[Gabriel-Trintinalia]

recheck

[gfukushima]

lgtm, @StefanBratanov might to want to have a look at this as well