Add cli option for passing a second key

[alexhulbert]

📝 Summary

• Replace single --ssh-pubkey-file with --host-ssh-pubkey-file and --container-ssh-pubkey-file flags
• Serve both pubkeys at /pubkey endpoint, separated by a newline
• Container pubkey is optional

⛱ Motivation and Context

• Now that the searcher provides their encryption password over an SSH to the CVM host (dropbear), we need to provide both keys to prevent an MITM attack
• The searcher can just copy the text into known_hosts as before

---

✅ I have run these commands

• make lint
• make test
• go mod tidy

[alexhulbert]

The lint doesn't work in CI but it works fine on my laptop

[Ruteri]

This server doesn't have to be specific to sandboxes — and the API actually is not still, simply returning a list of pubkeys.
However, the flags are extremely specific to sandboxes, I think needlessly so. Would it work to simply allow a list of files be passed?