This diff adds a `base64` crate to the Cargo Tunnelbroker dependencies.
`base64` functions will be used in the following D5945 in a `verify_signed_string` function.
Linear task: [[ https://linear.app/comm/issue/ENG-2492/removing-cryptopp-from-the-tunnelbroker-and-use-rust-openssl-crate | ENG-2492 ]]