Add a HasTypeProxy constraint to getVerificationKey

[erikd]

Changelog

- description: Add a HasTypeProxy constraint to getVerificationKey
  # no-api-changes: the API has not changed
  # compatible: the API has changed but is non-breaking
  # breaking: the API has changed in a breaking way
  compatibility: compatible
  # feature: the change implements a new feature in the API
  # bugfix: the change fixes a bug in the API
  # test: the change fixes modifies tests
  # maintenance: the change involves something other than the API
  # If more than one is applicable, it may be put into a list.
  type: maintenance

Context

This constraint is needed in cardano-cli.

Checklist

• Commit sequence broadly makes sense and commits have useful messages
• The change log section in the PR description has been filled in
• New tests are added if needed and existing tests are updated. These may include:
  • golden tests
  • property tests
  • roundtrip tests
• The version bounds in .cabal files are updated
• CI passes. See note on CI. The following CI checks are required:
  • Code is linted with hlint. See .github/workflows/check-hlint.yml to get the hlint version
  • Code is formatted with stylish-haskell. See .github/workflows/stylish-haskell.yml to get the stylish-haskell version
  • Code builds on Linux, MacOS and Windows for ghc-8.10.7 and ghc-9.2.7
• The changelog section in the PR is updated to describe the change
• Self-reviewed the diff

Note on CI

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You will need to get someone with write privileges. Please contact IOG node developers to do this
for you.