This diff introduces a signed string verification function to use in the "new session" creation process in the Tunnelbroker API. This function will be used in the following D5947 to verify the signed string by the public key provided to make sure the client "own" keys.
Linear task: [[ https://linear.app/comm/issue/ENG-2492/removing-cryptopp-from-the-tunnelbroker-and-use-rust-openssl-crate | ENG-2492 ]]