Active peers last advertized head RPC

This MR aims to bring a new RPC that display, for each active peers connected to the node, its peer_id and block_hash+block_level of the last adevrtized head.

It should display something like

$curl -s http://127.0.0.1:8732/chains/main/active_peers_heads | jq
{
  "active_peers_heads": [
    {
      "peer_id": "idts56eVgbK6gjvcrW9jdFSL9fcj4d",
      "block_hash": "BLgiEgr4LFUBdB5e6mvHSVhDbewBPPhPnBm35ARvfgLjuuRuAHR",
      "block_level": 1725631
    },
    {
      "peer_id": "idsorXP8nfvSi6kr57CqqpX7URe2tB",
      "block_hash": "BLgiEgr4LFUBdB5e6mvHSVhDbewBPPhPnBm35ARvfgLjuuRuAHR",
      "block_level": 1725631
    },
    {
      "peer_id": "idt6U1SmJGPFEZy11GS1JqQtufFDFW",
      "block_hash": "BLgiEgr4LFUBdB5e6mvHSVhDbewBPPhPnBm35ARvfgLjuuRuAHR",
      "block_level": 1725631
    },
    {
      "peer_id": "idsZkDkQusT1qYYXZ8Dz1EgEhXyNJT",
      "block_hash": "BLgiEgr4LFUBdB5e6mvHSVhDbewBPPhPnBm35ARvfgLjuuRuAHR",
      "block_level": 1725631
    }
  ]
}

Manually testing the MR

Run the RPC against a node and enjoy.

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
• Document any change to the user interface, including configuration parameters (see node configuration)
• Provide automatic testing (see the testing guide).
• For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, CHANGES.rst at the root of the repository for everything else).
• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR