Make committee keys able to sign transactions

[smelc]

Changelog

- description: |
    Make committee keys able to sign transactions
# 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
  # - improvement    # QoL changes e.g. refactoring
  # - 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 #586

How to trust this PR

• The reproducing procedure from cardano-cli cannot sign with ConstitutionalCommittee Cold/Hot ExtendedSigningKey_ed25519_bip32 #586 works now
• Tests are added

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

[gitmachtl]

This is not working:

#586 (comment)