This diff introduces the unit test for the `sign_string_with_private_key` function. This function gets the string to be signed and the private key and returns a sha256 signature. We will use this function in the Tunnelbroker client's new session creation because we need to [[ https://www.notion.so/commapp/Tunnelbroker-028bdd578e4c487fb570dbb862952ec6#9758ddb84edd4a9f86ad07a9d8a4b39e | provide the signed string to the Tunnelbroker ]] to prove the ownership of the keys.
In this unit test, we will generate the keypair and sign the string, then we will verify the signature by the OpenSSL verifier.
Linear task: [[ https://linear.app/comm/issue/ENG-1333/implement-native-client-for-the-tunnelbroker-api-using-rust | ENG-1333 ]]