[services/commtest] Adding of openssl, base64, anyhow to dependencies

Summary:
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).

Test Plan: The commtest app is successfully built and the tests are successfully passed in the following D5931.

Reviewers: jon, varun, ashoat

Reviewed By: jon, ashoat

Subscribers: ashoat, tomek, atul

Differential Revision: https://phab.comm.dev/D5926