This diff adds the following Rust crate dependencies to the `commtest` API testing app:
- OpenSSL: We need it to make a signature for the string by the private key (in D5927), and generate keys as well (D5931).
- Base64: We are representing the signed string in a `base64` to send it to the Tunnelbroker (in D5927).
- Anyhow: To be flexible in error types returns in utility functions we should use an `Anyhow` error type (in D5929).