[CO-410] Introduce NetworkMagic

[intricate]

Description

Building on the previous PR (#3715), we add the NetworkMagic datatype, which allows us to discriminate addresses originating from different networks.

In order to make this commit as simple and easy-to-review as possible, I stub out the added arguments of type NetworkMagic with a dummy variable: fixedNM. This puts off the (fairly involved) work of threading NetworkMagic values from the nearest occurrence of ProtocolMagic to where said values are needed.

The remainder of the work will come in a separate commit.

Linked issue

https://iohk.myjetbrains.com/youtrack/issue/CO-410

Type of change

• [~] 🐞 Bug fix (non-breaking change which fixes an issue)
• 🛠 New feature (non-breaking change which adds functionality)
• [~] ⚠️ Breaking change (fix or feature that would cause existing functionality to change)
• [~] 🏭 Refactoring that does not change existing functionality but does improve things like code readability, structure etc
• [~] 🔨 New or improved tests for existing code
• [~] ⛑ git-flow chore (backport, hotfix, etc)

Developer checklist

• I have read the style guide document, and my code follows the code style of this project.
• If my code deals with exceptions, it follows the guidelines.
• I have updated any documentation accordingly, if needed. Documentation changes can be reflected in opening a PR on cardanodocs.com, amending the inline Haddock comments, any relevant README file or one of the document listed in the docs directory.
• [~] CHANGELOG entry has been added and is linked to the correct PR on GitHub.
  ^ there is no 1.3.1 section in the CHANGELOG, and this is dev work on a release branch, so I'm skipping this.

Testing checklist

• [~] I have added tests to cover my changes.
• All new and existing tests passed.
  ^ no tests were added because these changes introduce new capabilities which are not used yet. Thus the behavior is the same, and the existing tests should be adequate.

[mhuesch]

Looks good overall. We should remove those TODOs. The other comments are optional.

What's up with wallet/test/wallet-db-1.1.1/open.lock? It seems it was added as a 0-line file in this PR.

[erikd]

Hydra seemed to have screwed up the building of this PR so I rebased this and force pushed.

[mhuesch]

Merging before any conflicts arise!