feat: multi-threaded verify

[spiral-ladder]

make verification of multiple aggregate signatures multithreaded above a certain number of elements.

[nazarhussain]

Initiating main thread work, before spawning workers enforces that main thread work is finished before child threads started, defeats parallelism.

[nazarhussain]

Suggested change

	// Main thread does worker 0's work
	workerFn(&contexts[0]);
	for (0..n_workers - 1) |t| {
	threads[t] = std.Thread.spawn(.{}, workerFn, .{&contexts[t + 1]}) catch return BlstError.VerifyFail;
	spawned += 1;
	}
	for (0..n_workers - 1) |t| {
	threads[t] = std.Thread.spawn(.{}, workerFn, .{&contexts[t + 1]}) catch return BlstError.VerifyFail;
	spawned += 1;
	}
	// Main thread does worker 0's work in parallel with spawned threads
	workerFn(&contexts[0]);

[spiral-ladder]

closing this in favour of ChainSafe/lodestar-z#231