Add a CLI command for exiting validators

[paulhauner]

Description

Presently we don't have a way for validators to submit a VoluntaryExit. Once #1569 is merged, I think we should create a tool that uses the BN pool/voluntary_exits endpoint.

I expect it would look something like this:

lighthouse account validator exit --beacon-node http://localhost:5052 --keystore my-keystore.json --password-file my.pass

• If --password-file is not supplied, it should be requested interactively.
• --beacon-node can have a default value.

Additional Info

Here's a ready-made function to do a HTTP request to a beacon node:

lighthouse/common/eth2/src/lib.rs

Lines 491 to 507 in f14e58b

	/// `POST beacon/pool/voluntary_exits`
	pub async fn post_beacon_pool_voluntary_exits(
	&self,
	exit: &SignedVoluntaryExit,
	) -> Result<(), Error> {
	let mut path = self.eth_path()?;
	path.path_segments_mut()
	.map_err(|()| Error::InvalidUrl(self.server.clone()))?
	.push("beacon")
	.push("pool")
	.push("voluntary_exits");
	self.post(path, exit).await?;
	Ok(())
	}

[pawanjay176]

Taking this one