Add validator voluntary exit endpoint

[jimmygchen]

Addresses #57.

I've incorporated inputs from the discussions in the above issue and the R&D #api channel.

[jimmygchen]

Converting back to draft as I just realised that I need to update the flows and use-cases docs.

[jimmygchen]

I've just went through the docs and not sure if extra info is need for this endpoint - I think the description and spec of the endpoint seems adequate, but please let me know otherwise!

[rkapka]

Copying from Discord:

I don't really see the purpose of the confirmation query parameter. In CLI apps we usually issue a warning after the user initiates an operation, and confirming is a second step. The warning is there to allow the user to back out from the operation. The semantics of an API query parameter are different. We only have a single step, so you either go through with the operation when including the query param or you don't when not including it - the param doesn't add any value. I don't ever see a reason for anyone to invoke the endpoint without the query parameter because it will just fail (which, again, doesn't work that way in a CLI app). In my opinion it is redundant.

[dapplion]

Agree with the reasoning above, I would drop confirm param too

[jimmygchen]

Agree with the reasoning above, I would drop confirm param too

Thanks @dapplion - there's some discussions on this topic in the #api channel on R&D, and it looks like the consensus so far is to drop the query param. The alternative proposal is to change the API, so that it returns a SignedVoluntaryObject and let the user send it to the Beacon API, making it a two step process. This would prevent a simple curl from triggering an irreversible operation, and also gives us flexibility to prepare an exit message ahead of time.

If there's no strong objections I'll update this PR shortly.

[jimmygchen]

@dapplion and Adrian also raised that web3signer implements keymanager API but can't fetch the current epoch like VC does, so we'll have to make epoch a mandatory param.

[jimmygchen]

Updated the endpoint based on discussion in api channel:

• Return SignedVoluntaryExit object in response
• Remove confirm query parameter
• Make epoch query parameter mandary

[jimmygchen]

Discussion from Discord (R&D, #api) - given that remote signers already supports signing voluntary exits, and does not implement the full keymanger API, this endpoint doesn't seem necessary for remote signer.

I've made the epoch query parameter optional, as validator client has knowledge of the current epoch and can set this as default. Also updated the tag description to explicitly mention sections that are "Validator Client only" - Fee recipient, Gas Limit & Voluntary exits.

[rolfyone]

LGTM