This diff introduces adding of the C++ function which is exposed to the Rust using the CXX bridge to get the string to verify from the DynamoDB database by the `deviceID`.
To create a new session [[ https://www.notion.so/commapp/Tunnelbroker-028bdd578e4c487fb570dbb862952ec6#9758ddb84edd4a9f86ad07a9d8a4b39e | client first gets the verification string ]] from the server and then sign it with the private key. The randomly generated string is saved into the DynamoDB table for the corresponding `deviceID` during the request. To verify the string in the following D5947 and Rust verify function in D5945 we should get this string.
Linear task: [[ https://linear.app/comm/issue/ENG-2492/removing-cryptopp-from-the-tunnelbroker-and-use-rust-openssl-crate | ENG-2492 ]]