Validator identity pubkey <-> keybase account link

[mvines]

We're planning on using keybase as the source of validator's public profile. Ideally we pull the validator's public key from their profile as well.

In the short term this could be accomplished by asking validators put their validator pubkey in a public file on keybase, for example: https://keybase.pub/mvines/solana-validator.pubkey

This pubkey can then be pulled programmatically for use by the blockexplorer.

The specific tasks here are:

• [x] Document the process for publishing a pubkey on keybase. This probably ends up in the Testnet Participation Guide after #4970 lands
• [x] Consider adding verification in the solana-validator-info tool that the validator's pubkey is present, and guide the user on how to add it if not
• [x] A tiny tool that ingests a list of keybase ids and outputs a list of matching validator pubkeys for use by #4960

[mvines]

Instead of https://keybase.pub/mvines/solana-validator.pubkey, we can use the pubkey in the file name itself: https://keybase.pub/mvines/solana_pubkey_<PUBKEY>. This enables us to do a single http HEAD to determine if the pubkey is published, no need to download and parse a file. This would also allow a Keybase account to effortlessly support multiple pubkeys