[16/n][Services] Tunnelbroker - Crypto tools.

Summary:
Full stack:
D2809 D2875 D2752 D2904 D2759 D2760 D2808 D2762 D2876 D2769 D2877 D2807 D2814 D2870 D2888 D2915 D2976 D2977 D2978

Cryptography tools.

The main purpose is to verify the signature by the verifySignature() using the public RSA key.

Test Plan: Verified by passing the RSA public key, message, and signature to the function and checking the results.

Reviewers: palys-swm, ashoat, karol-bisztyga

Reviewed By: palys-swm, ashoat, karol-bisztyga

Subscribers: benschac, ashoat, palys-swm, Adrian, atul, karol-bisztyga

Differential Revision: https://phabricator.ashoat.com/D2915