offline-phase: lowgear: shared-random: Implement F_rand subprotocol

[joeykraut]

Purpose

This PR implements the $\mathcal{F}_\text{rand}$ subprotocol used to generated shared random values in the offline phase. This is done in a few simple steps:

1. Each party samples random values locally
2. Each party commits to their random values
3. Each party reveals their random values, the counterparty then checks the commitment
4. The parties sum their sampled shares to construct the shared randomness

Testing

• Unit tests pass
• Tested the rand subprotocol in unit