[pubsub]: Node ID private key as parameter.

[akru]

Motivation

According to https://github.com/airalab/robonomics/issues/46 is better to dedicate node identity and store it into a file for long term usage.

Steps

• [ ] Add into identity module function to save/load peer private key from file.
• [ ] Move peer identity into Gossipsub::new() function parameter.

[Vourhey]

Is it still relevant? @akru