Add cardano-blueprint submodule and move handshake cddl to it

[ch1bo]

Description

This demonstrates how we could move the CDDL to a common repository and test against it from multiple implementations of the ouroboros protocols.

Git submodules are obviously only one way of managing this dependency.

Checklist

Quality

• [x] Commit sequence makes sense and have useful messages, see ref.
• [x] New tests are added and existing tests are updated.
• [x] Self-reviewed the PR.

Maintenance

• [ ] Linked an issue or added the PR to the current sprint of ouroboros-network project.
• [ ] Added labels.
• [ ] Updated changelog files.
• [ ] The documentation has been properly updated, see ref.

[ch1bo]

@coot I'd love to use this PR as a basis for discussion on how we could move the CDDLs into a place where they are more easily consumed by other implementations.

[coot]

@ch1bo, such an approach will work, but first, this should be discussed in a wider forum, e.g. with consensus & ledger teams. We should not corner them with a decision like that.

I haven't used git-submodules extensively, but enough to know it can be painful. Are there other possible approaches which avoid git-submodules?

Can one use tags instead of git-hashes? It would be much nicer to say node-to-node-v14, isn't it?