[F3] Implement F3.Finalize RPC method

[hanabi1224]

Summary

Originally part of #4775

        // Finalize marks the tipset that corresponds to the given key as finalised
	// beyond which no forks are allowed to occur. The finalised tipset overrides the
	// head tipset if it is not an ancestor of the current head.
	//
	// See GetHead.
	Finalize(context.Context, gpbft.TipSetKey) error

func (ec *ecWrapper) Finalize(ctx context.Context, key gpbft.TipSetKey) error {
	if !ec.Checkpoint {
		return nil // Nothing to do; checkpointing is not enabled.
	}
	tsk, err := toLotusTipSetKey(key)
	if err != nil {
		return err
	}
	if err = ec.Syncer.SyncCheckpoint(ctx, tsk); err != nil {
		return xerrors.Errorf("checkpointing finalized tipset: %w", err)
	}
	return nil
}

Completion Criteria

• Implement F3.Finalize RPC method

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