Add REST implementation for ValidatorStatus

[nalepae]

What type of PR is this?

Feature

Related gitHub issue:

#11580

What does this PR do? Why is it needed?

This adds an implementation for the ValidatorStatus validator client method by calling the Beacon Node REST API instead of the gRPC one.

Notes for reviewers:

Eth1DepositBlockNumber & DepositInclusionSlot:

In gRPC implementation, the beacon node returns Eth1DepositBlockNumber & DepositInclusionSlot.
As far as I know, the Beacon API does not offers easy way to retrieve these data.
Unless I'm mistaken, these data are not used in the Validator Client.
I chose to skip them. Please tell me if this decision is not valid.

Ordered validators

I assumed the call to /eth/v1/beacon/states/head/validators returns a sorted (by index) list of validators.
However, I was not able to find such a sort requirement in the specification, even if Prysm, Teku and Lighthouse (did not test Nimbus) seem to respect this sort standard.

The advantage of this assumption is, in order to retrieve the last active validator, we just have to get the last element of the list. (O(1))
Please tell me if such an assumption is not valid.

Position in active queue

I tried to mimic the behaviour of https://github.com/prysmaticlabs/prysm/blob/develop/beacon-chain/rpc/prysm/v1alpha1/validator/status.go#L272.

This implementation assumes that the position in active queue for a given validator is the difference between the index of the validator and the highest index in the set of currently active validators.

If we consider the following case:

validatorIndex | validatorStatus
-------------------------------
1000           | active_ongoing
1001           | active_ongoing
1002           | exited_slashed
1003           | pending_queued
1004           | pending_queued    <--- our validator

Then:

• the real position in active queue for our validator will be 1004 - 1002 = 2
• the indicated position in active queue for our validator will be 1004 - 1001 = 3

(But I think it's still a good proxy)