Accelerate signer fixtures creation in tests with pre-computation

[jpraynaud]

Why

The creation of test fixtures for the signers takes a long time due to the underlying cryptographic operations. We want to reduce the time of computation of these fixtures with some caching or pre-computation.

What

Implement pre-computation of signer fixtures with deterministic randomization.

How

• Pre-compute and cache in a repository file the 10 most common signer fixtures that we use
• Create a cache as a const map with a build script (read a pre-generated JSON file with cache key (party_index + party_id_seed) and cache entry the fixture generated by the existing builder)
• When computing the fixtures, use the cache
• Create a test (or very light binary) to pre-generate the JSON file with the cache keys and cache entries (and document it)