CSUB-336 weighted VRFs

[CAGS295]

Description of proposed changes:
This is an early implementation of the basics. I will be rebasing without notice until the interface is stabilized.
With this, you can generate a Transcript that you can use along a sr25519 public to generate a provable seed. The seed is extended with personal data to generate a random number. The provable seed can be verified publically and the result of such proof is the extended provable seed that can be used directly to generate a random number. The PR also contains a pseudo-probabilistic model to generate a threshold based on the stake and a sample ratio hyperparameter.

I also included rudimentary positive tests.

Non.exhaustive TODOs:

• Stabilize interface.
• Error handling.
• specialize functions.
• better parameters at the runtime_interface boundaries.

Practical tips for PR review & merge:

• All GitHub Actions report PASS
• Newly added code/functions have unit tests
  • Coverage tools report all newly added lines as covered
  • The positive scenario is exercised
  • Negative scenarios are exercised, e.g. assert on all possible errors
  • Assert on events triggered if applicable
  • Assert on changes made to storage if applicable
• Modified behavior/functions - try to make sure above test items are covered
• Integration tests are added if applicable/needed

[github-actions]

For full LLVM coverage report click here!

[codecov-commenter]

Codecov Report

Merging #831 (cde7be9) into dev (af5da66) will increase coverage by 0.19%.
The diff coverage is 87.37%.

@@            Coverage Diff             @@
##              dev     #831      +/-   ##
==========================================
+ Coverage   78.26%   78.45%   +0.19%     
==========================================
  Files          68       68              
  Lines       10724    10856     +132     
==========================================
+ Hits         8393     8517     +124     
- Misses       2331     2339       +8     

Impacted Files	Coverage Δ
pallets/difficulty/src/lib.rs	93.05% <ø> (ø)
primitives/src/lib.rs	93.33% <ø> (ø)
runtime/src/lib.rs	24.13% <0.00%> (+0.35%)	⬆️
primitives/src/vrf.rs	94.09% <89.00%> (-3.11%)	⬇️
sha3pow/src/lib.rs	94.38% <100.00%> (ø)
node/src/main.rs	16.66% <0.00%> (ø)
test/runtime-utils/src/lib.rs	100.00% <0.00%> (ø)

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[atodorov]

Minor changes requested.

I think I need some more context here before I can review the tests further.

[atodorov]

Requesting changes to names of variables to make the tests more intuitive to understand.

[nathanwhit]

First pass at review, just have a few questions

[nathanwhit]

LGTM