Allow the VC to listen on 0.0.0.0 with a flag

[paulhauner]

Description

Presently, the VC doesn't accept the --http-address field for this reasoning:

lighthouse/validator_client/src/cli.rs

Lines 121 to 128 in 3b600ac

	* Note: there is purposefully no `--http-address` flag provided.
	*
	* The HTTP server is **not** encrypted (i.e., not HTTPS) and therefore it is unsafe to
	* publish on a public network.
	*
	* We restrict the user to `127.0.0.1` and they must provide some other transport-layer
	* encryption (e.g., SSH tunnels).
	*/

This restriction has showed to be very annoying for Docker setups; you can't access the container from another container, even if you never intend to expose it to the internet. @dapplion raised this most recently.

I propose we make the following changes:

• Add the --http-address flag to the VC.
• Add the ---unencrypted-http-transport flag, and require it to be present if --http-address is used.
  • In the help text for this flag we should clearly explain why it's unsafe.

P.S., Don't forget to update the VC CLI tests! https://github.com/sigp/lighthouse/blob/stable/lighthouse/tests/validator_client.rs

[macladson]

Closed by #2467