make voku package optional

[jasverix]

This is:

• a bugfix
• a new feature
• [ x ] refactoring
• additional unit tests

Checklist:

• Changes are covered by unit tests
  • Changes are covered by existing unit tests
  • New unit tests have been added
• [ x ] Code style is respected
• Commit message explains why the change is made (see https://github.com/erlang/otp/wiki/Writing-good-commit-messages)
• CHANGELOG.md contains a short summary of the change and a link to the pull request if applicable
• Documentation is updated as necessary

Why this change is needed?

Because the voku/portable--utf8 has a bootstrap which runs an ini_set() on all requests. #3954

[oleibman]

Closing - we cannot make this optional without exposing our users to security issues. Discussion can continue in issue #3954.