SSL/TLS encryption everywhere

Context

BuildGrid currently expose an insecure/unencrypted end-point. SSL/TLS encryption should be supported for any gRPC communications.

Task Description

• Add relevant CLI options for client/server certificate and private key where it make sense.
• Have sensible default search paths for these files if CLI options are not specified.
• Keep around an --allow-insecure option on server-side.
• Encrypt gRPC communications.