Enable withdrawals

[AgeManning]

We have built a new endpoint that allows a single POST request to voluntary exit validators. See here: https://github.com/sigp/lighthouse/issues/4117

This will enable the UI to very easly exit validators.

We should display a big warning sign, along the lines of "ARE YOU SURE YOU WANT TO EXIT" as well as like a type "Exit my Validator" before the UI does it. This is a non-reversible, permanent change to the validator.

We may even want to consider potentially having some time-lock or password such that if a user leaves their computer in a public space, someone can't come along and just exit their validators.

Maybe people could enter an optional password, that they need to re-enter after 10 mins of being idle or something before enabling destructive actions like withdrawing a validator or converting BLSToExecution changes.

[jimmygchen]

The endpoint has been implemented in https://github.com/sigp/lighthouse/pull/4119

The endpoint returns a SignedVoluntaryExit object, which can be submitted to the Beacon API.

API spec here

[jimmygchen]

We may even want to consider potentially having some time-lock or password such that if a user leaves their computer in a public space, someone can't come along and just exit their validators

This was also briefly discussed today, we may want to consider prompting user to enter password every time user wants to initiate an exit (@realbigsean's suggestion).