handshake: implement (togglable) pop verification

[ozwaldorf]

Adds pop verification to handshake. Client signs in the hello frame with a nonce to prove possession of the key and authenticate.

• Outlines session expiration in the handshake frame. The pod signatures would include a timestamp that must fall before the expiration.
• added --pop test flag to the fetcher client. Working, but fails later on for the blockstore sdk utils and the new b3fs integration
• added handshake.verify config field for enabling the pop verification and sending back the handshake response (can enable when testing, but testnet can still ignore it until we have the full pod flow)