Add public_key hash to /v2/info endpoint

[wileyj]

It would be nice to know the public key of a hosted node from a common endpoint.
For example, consider this possible /v2/info response:

{
  "peer_version": 402653189,
  ...
  "unanchored_seq": null,
  "exit_at_block_height": null,
  "public_key": "02da7a464ac770ae8337a343670778b93410f2f3fef6bea98dd1c3e9224459d36b"
}

Providing this would be a welcome addition since it would allow a stacks-blockchain operator to decentralize the "seed" nodes from Hiro, or at least provide additional options for seed nodes outside of the ones Hiro provides in https://github.com/stacks-network/stacks-blockchain/blob/master/testnet/stacks-node/conf/mainnet-follower-conf.toml#L5.

Future application could be some org PR's an additional seed node to the linked config, or directly in the codebase with the config providing an override similar to bitcoin: https://github.com/bitcoin/bitcoin/blob/623745ca74cf3f54b474dac106f5802b7929503f/src/chainparams.cpp#L121-L129