Changes to Validator Client voluntary_exit endpoint response

[jimmygchen]

There is a non-compliance on the voluntary_exit endpoint response with the key manager API spec, and is being addressed in Lighthouse sigp/lighthouse#4679 (still under review)

This is a small change to add a data layer to the response, but it's a breaking change for Siren and response type needs to be updated.

Existing response

{"message":{"epoch":"196868","validator_index":"505597"},"signature":"0xhexsig"}

New response:

{"data":{"message":{"epoch":"196868","validator_index":"505597"},"signature":"0xhexsig"}}

It might be worth supporting both response types in the UI for a little while so users aren't force to upgrade to the latest Lighthouse version.

[rickimoore]

#220

addressed and merged in this pr