Add JSON schema checking functionality for validating gov action metadata

[palas]

Changelog

- description: |
    Added schema checking functionality for DRep registration, DRep update, and GovAction metadata, based on CIP-0100, CIP-0108, and CIP-0119. Also add functionality to check whether a certificate is for DRep registration or updating.
  type:
  - feature
  - compatible
  - test

Context

The objective is addressing this issue: IntersectMBO/cardano-cli#906

This PR adds 3 functions to the API that should be the only ones needed by the cardano-cli in order to implement this functionality:

• isDRepRegOrUpdateCert - Checks whether a certificate is for DRep registration or DRep update.
• validateDRepAnchorData - Checks whether a ByteString is valid JSON and conforms to CIP-0119.
• validateGovActionAnchorData - Checks whether a ByteString is valid JSON and conforms to CIP-0108

This PR is a requirement from PR #995 that implements the checking in cardano-cli and is based of the version of cardano-api in this PR.

Related PRs in CIPs repo:

• PR #950 - Adds @ before type field name
• PR #951 - Ensures CIP-0008 is in capital letters

How to trust this PR

Tests should give some assurance. They are taken from the CIPs repo, with some adaptations that I have already created PRs in CIPs repo for (see above).

The main thing to check here is the accuracy of the checks (are they enough, are they too much, are they right).

Could be useful to check the cardano-cli PR together with this one.

Checklist

• Commit sequence broadly makes sense and commits have useful messages
• New tests are added if needed and existing tests are updated. See Running tests for more details
• Self-reviewed the diff

[carbolymer]

But the CIPs 0119 and 0108 are in proposed state. Shouldn't we wait till they get accepted?

[Jimbo4350]

LGTM! Can you clean up the commit history and add links to the CIPs at the top of the modules?