configuration file for rust-proofs

[sidke]

Description

one should be allowed to configure rust-proofs through a configuration file
possibly can be read from the file and sent through to rust-proofs from go-filecoin via FFI

Acceptance criteria

• provide configuration to rust-fil-proofs without environment variables
• use a file accessible to gfc and pass through ffi
• have default configuration values
• have configuration profiles, sets of predefined values that can be chosen from instead of setting everything specifically

[porcuquine]

I think it's fine (possibly desirable) if env vars are also used to override — they just shouldn't be the sole method. Just mentioning for clarification.