offline-phase: lowgear: shared_bits: Implement shared bits generation

[joeykraut]

Purpose

This PR generates shared bits in the offline phase by consuming Beaver triplets. This is done by squaring a random value, opening it, inverting the opened value, then multiplying with the original random value. This gives a value in ${-1, 1}$ which can then be shifted using public operations into ${0, 1}$.

Testing

• Unit tests pass
• Tested the shared bits method, verified that the macs are correctly computed and the resulting values are in ${0, 1}$.