feat(core): wire up validatorapi Component subscribe/await/verify hooks

Adds the plumbing every subsequent submit/await handler needs without
implementing any of the unimplemented!() arms. Mirrors Charon's
core/validatorapi/validatorapi.go:196-256 (subscriber list + six
Register* hooks) plus :1352 (verifyPartialSig).

- New Component fields: subs, await_proposal_fn, await_agg_attestation_fn,
  await_sync_contribution_fn, await_agg_sig_db_fn, duty_def_fn,
  pub_key_by_att_fn. All Option<Arc<…>> so registration before the
  Component is shared in an Arc, then read-only thereafter.
- subscribe() wraps the user closure with a set-clone step so each
  subscriber receives its own ParSignedDataSet — matches Go's
  Subscribe clone-before-fanout at validatorapi.go:249-256.
- register_* methods replace any prior registration, matching Go's
  single-function input semantics.
- verify_partial_sig() honours insecure_test, looks up this node's
  public share from pub_share_by_pubkey, then delegates to
  pluto_eth2util::signing::verify. Unlike Go — which projects domain /
  epoch / message-root through the core.Eth2SignedData interface — the
  Rust hook takes those three values directly so we don't have to port
  the Eth2SignedData trait in this plumbing PR; submit handlers in PRs
  3-6 will derive the triple from their concrete signed-data wrapper.

Tests: subscribe fanout clones per subscriber; the six register hooks
all overwrite on re-register; unregistered hooks default to None;
verify_partial_sig accepts a real BLS signature, rejects a tampered
one, rejects an unknown DV pubkey, and short-circuits in insecure_test
mode.

Co-Authored-By: Bohdan Ohorodnii <35969035+varex83@users.noreply.github.com>

Author: varex83agent