Use selectStakeCredentialWitness instead of duplicating credential selection

[carbolymer]

Changelog

- description: |
    Use `selectStakeCredentialWitness` instead of duplicating credential selection
# 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

This PR requires:

• Expose stakeCredentialWitness function, which returns only stake credentials for the certificates requiring witnessing: delegation and deregistration cardano-api#383

Fixes:

• Cannot delegate Plutus stake address #297 (comment)

How to trust this PR

Highlight important bits of the PR that will make the review faster. If there are commands the reviewer can run to observe the new behavior, describe them.

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