Expose RNG or Seeded mechanisms outside of -testing

[mjbommar]

Problem Description

Downstream libraries or academic researchers using differential-privacy may desire to implement deterministic tests or substitute an alternative source of randomness, e.g., external random source.

However, RNG and seed-setting only appears to be implemented within algorithms::numerical-mechanisms-testing. These classes are not designed for external use and cannot be linked against in some contexts (e.g., bzl testonly).

Proposed Solution

1. Create a new RNGNumericalMechanism abstract class and non-testing RNGNumericalMechanismBuilder
2. Implement non-testing RNGXYZMechanism and, optionally, SeededXYZMechanism as convenience
3. Re-implement test SeededXYZMechanism classes with non-testing mechanisms
4. Optionally implement warning/notice for use
5. Implement safe downstream deterministic test cases, e.g., in PyDP

Alternative

Rely on asymptotic test cases or non-reproducible examples/research.