feat(eth2util): implement keymanager

[iamquang95]

Should fix: #166

[emlautarom1]

LGTM, I would only add that it would be nice to have integration tests that ping a real CL. You can copy the pattern in eth2_api for that.

Also, given the scope of the feature it does not really make sense, but in the future we could extend the eth2_api crate with the schema defined in https://ethereum.github.io/keymanager-APIs/ since it contains the endpoints and types used by this feature.

[iamquang95]

Also, given the scope of the feature it does not really make sense, but in the future we could extend the eth2_api crate with the schema defined in https://ethereum.github.io/keymanager-APIs/ since it contains the endpoints and types used by this feature.

It's maybe a bit extra for the current scope since we only need 1 api from that. Will add it later in the future when we have more API usages