Cardano blueprint handshake tests

[ch1bo]

This demonstrates how we could test against test vectors in a common repository from multiple implementations of the ouroboros protocols.

For that matter, simple serialization roundtrip tests against the handshake protocol cddl are added in this PR.

The cardano-blueprint is added as a submodule, but tests using data from it are gated by a blueprint feature flag.

This means, new contributors would experience this workflow:

• git clone git@github.com:txpipe/pallas.git && cd pallas
• cargo test -> works, while tests using cardano-blueprint are not run
  • Contributor starts hacking on things
  • NOTE: Any changes are checked in CI including the blueprint tests!
• After a CI failure or when deliberately wanting to run the blueprint tests:
  • cargo test --features blueprint -> fails because submodule not checked out
  • git submodule update --init checks out submodule
  • cargo test --features blueprint -> works now

[ch1bo]

Happy to remove the flake.nix stuff from this PR.

Also not sure yet about the test data this uses. Right now it's just the first 4 messages created by the Haskell implementation (on one run, it's probabilistic)

[ch1bo]

@scarmuega Any opinion on this general approach?

[ch1bo]

Moved the nix bits into an own repository I can easily pull into my workflow for pallas (https://github.com/ch1bo/rust-flake)

I will look into this cargo xtask stuff to make the submodule invisible to first time contributors who just want to git clone and cargo test pallas.

[ch1bo]

@scarmuega: @KtorZ and me discussed this briefly and we thought putting the cardano-blueprint tests behind a feature flag would work too? What do you think?

[ch1bo]

@scarmuega This should be ready now. The CI action seems not to run on pull_request (I changed that in this PR too)

I also updated the PR description to explain how the feature flag works.

[ch1bo]

Was merged by closing #638