ssv-network: SSV-node API support

[iurii-ssv]

This PR should allow a commit-boost operator to use the API of their own SSV node (instead of relying on a 3rd party API that is currently available at api.ssv.network).

The SSV node API looks like this (it is different from the API-endpoint exposed by api.ssv.network service):

• request example:

curl --request GET \
  --url {ssv_node_base_url}/v1/validators \
  --header 'Content-Type: application/json' \
  --data '{
  "operators": [1]
}'

• response example:

{
  "data": [
    {
      "public_key": "aa370f6250d421d00437b9900407a7ad93b041aeb7259d99b55ab8b163277746680e93e841f87350737bceee46aa104d",
      "index": "1311498",
      "status": "active_ongoing",
      "activation_epoch": "273156",
      "exit_epoch": "18446744073709551615",
      "owner": "5e33db0b37622f7e6b2f0654aa7b985d854ea9cb",
      "committee": [
        1,
        2,
        3,
        4
      ],
      "quorum": 0,
      "partial_quorum": 0,
      "graffiti": "",
      "liquidated": false
    }
  ]
}

See also the original implementation PR (the old way)

[jclapis]

I did a little cleanup and modified the SSV setup so now there are 2 separate parameters: ssv_node_api_url for the local node, and ssv_public_api_url for the 3rd-party public API server we previously used. Queries will try the local node first, and only use the public API as a fallback if the local node fails, I also added unit tests to confirm that the local node request routine works properly according to the above example, and to confirm the failover works transparently.

[iurii-ssv]

Awesome! Looks good to me.