External endpoint: /peers/connected

[uwiger]

See issue #3357

There is an internal endpoint, /debug/peers, which basically returns the same thing (at least the pubkeys of connected peers).
I added an external endpoint which returns an array of objects, currently containing pub_key, host and port of the relevant peers.

Example, from the new test aehttp_integration_SUITE:get_connected_peers/1:

connected_peers (dev1) = [{default,
                              {ok,200,
                                  #{<<"connected_peers">> =>
                                        [#{<<"host">> => <<"localhost">>,
                                           <<"port">> => 3025,
                                           <<"pub_key">> =>
                                               <<"pp_23YdvfRPQ1b1AMWmkKZUGk2cQLqygQp55FzDWZSEUicPjhxtp5">>},
                                         #{<<"host">> => <<"localhost">>,
                                           <<"port">> => 3035,
                                           <<"pub_key">> =>
                                               <<"pp_2M9oPohzsWgJrBBCFeYi3PVT4YF7F2botBtq6J1EGcVkiutx3R">>}]}}},
                          {all,
                              {ok,200,
                                  #{<<"connected_peers">> =>
                                        [#{<<"host">> => <<"localhost">>,
                                           <<"port">> => 3025,
                                           <<"pub_key">> =>
                                               <<"pp_23YdvfRPQ1b1AMWmkKZUGk2cQLqygQp55FzDWZSEUicPjhxtp5">>},
                                         #{<<"host">> => <<"localhost">>,
                                           <<"port">> => 3035,
                                           <<"pub_key">> =>
                                               <<"pp_2M9oPohzsWgJrBBCFeYi3PVT4YF7F2botBtq6J1EGcVkiutx3R">>}]}}},
                          {inbound,
                              {ok,200,
                                  #{<<"connected_peers">> =>
                                        [#{<<"host">> => <<"localhost">>,
                                           <<"port">> => 3025,
                                           <<"pub_key">> =>
                                               <<"pp_23YdvfRPQ1b1AMWmkKZUGk2cQLqygQp55FzDWZSEUicPjhxtp5">>},
                                         #{<<"host">> => <<"localhost">>,
                                           <<"port">> => 3035,
                                           <<"pub_key">> =>
                                               <<"pp_2M9oPohzsWgJrBBCFeYi3PVT4YF7F2botBtq6J1EGcVkiutx3R">>}]}}},
                          {outbound,
                              {ok,200,
                                  #{<<"connected_peers">> =>
                                        [#{<<"host">> => <<"localhost">>,
                                           <<"port">> => 3025,
                                           <<"pub_key">> =>
                                               <<"pp_23YdvfRPQ1b1AMWmkKZUGk2cQLqygQp55FzDWZSEUicPjhxtp5">>},
                                         #{<<"host">> => <<"localhost">>,
                                           <<"port">> => 3035,
                                           <<"pub_key">> =>
                                               <<"pp_2M9oPohzsWgJrBBCFeYi3PVT4YF7F2botBtq6J1EGcVkiutx3R">>}]}}}]

I added a three-node configuration for the peer_endpoints test group, in preparation for further tests. Also, the peer pool persistence issue (#3356) may warrant other parameters for each peer, which could be added to the result.

[gorbak25]

Looks good but this PR doesn't resolve this issue.
What we need besides this PR is a /peers/all_peers endpoint(can be internal) which lists all peers in the pool - even the disconnected ones. Nodes might be connected only to a handful of peers.

[uwiger]

What we need besides this PR is a /peers/all_peers endpoint(can be internal)

So taking /debug/peers as a starting point, what would you like to add?
It lists all connected peers, all inbound, all outbound and all blocked. Just the pubkeys.

Looking at the available peers api, we could also provide available_peers (verified | unverified | standby | hostnames), where hostnames are the ones that failed to resolve and are pending retry.

[gorbak25]

1. This endpoint should be disabled unless explicitly enabled by a user as it might be costly and might decrease the privacy of the network - this should only be enabled by nodes operated for monitoring the peers.
2. Exposing data from aec_peers:connected_peers is useless for the intended usecase
   

[uwiger]

Not needed/desired. Closing.