conway governance hash: add option to write hash to file (--out-file)

[smelc]

Changelog

- description: |
    Add `--out-file` flag to `conway governance hash` command, so that it can write its result to a file (instead of only to stdout)
# 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 #446

How to trust this PR

Execute cabal run cardano-cli -- conway governance hash --text foo --out-file out and witness out gets created

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