Add per subnet subscribers target for dynamic peer requirements

[zilm13]

PR Description

We have flag and configurable variable:
--Xp2p-target-subnet-subscriber-count which currently defaults to 2. It's required by the spec. It's very strange, but it's required by the spec. It checks the number of subnet subscription and if there is any, it will return it's value. So with default 2 it requires 2 total peers for any number of subscriptions. No subscriptions - 0 peers, 1,2,3,100 subnets - 2 peers total.

I've introduced flag --Xp2p-target-per-subnet-subscriber-count which is a configuration how many peers are required for each subnet. It's goal is to dynamically require additional peers depending on the node subnet subcriptions. So if say, you are a supernode with 128 subnets, you require 128 peers, if you need 8 custody subscriptiions, you require 8 peers. It's an experimental flag and defaults to unset so it will not affect any current behavior.

Fixed Issue(s)

Documentation

• I thought about documentation and added the doc-change-required label to this PR if updates are required.

Changelog

• I thought about adding a changelog entry, and added one if I deemed necessary.

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Note

Medium Risk
Changes peer requirement calculation used by peer selection to optionally scale with the number of subscribed subnets, which can affect target peer counts and network connectivity behavior. Default remains unset so existing behavior is preserved unless the new hidden flag is enabled.

Overview
Introduces a new hidden config/CLI option --Xp2p-target-per-subnet-subscriber-count (stored as OptionalInt in P2PConfig) to optionally specify a per-subnet subscriber target.

Updates PeerSubnetSubscriptions.getSubscribersRequired() to account for this per-subnet target by computing total subscriber shortfall across relevant attestation, sync committee, and data column sidecar subnets, and taking the max of that and the existing global/subnet-minimum based requirement.

Wires the new config through Eth2P2PNetworkBuilder into PeerSubnetSubscriptions.create, updates test fixtures, and adds/updates tests and YAML config parsing to cover the new setting.

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