Create toJSON instance for "query drep-state" output

[smelc]

Changelog

- description: |
    Create toJSON instance for `query drep-state` output. Haskell users can know use this type to parse back `query drep-state`'s output to a Haskell value automatically.
# uncomment types applicable to the change:
  type:
  - feature        # introduces a new feature
  # - breaking       # the API has changed in a breaking way
  - compatible     # the API has changed but is non-breaking
  # - optimisation   # measurable performance improvements
  # - refactoring    # QoL changes
  # - bugfix         # fixes a defect
  # - test           # fixes/modifies tests
  # - maintenance    # not directly related to the code
  # - release        # related to a new release preparation
  # - documentation  # change in code docs, haddocks...

Context

Fixes #606

How to trust this PR

• Use this version of cardano-node: cardano-testnet: test some queries with a golden file cardano-node#5943
• Execute the test running query drep-state with RECREATE_GOLDEN_FILES=1 cabal test cardano-testnet-test --test-options '-p "/CliQueries/"'. Observe that the test with the golden file still passes.

Checklist

• Commit sequence broadly makes sense and commits have useful messages
• Self-reviewed the diff