find ruby gems & make coverage conditional on BUILD_COVERAGE

Summary:
new finding tech:
- find_gem function configures gem-specific FindFoo files and wraps around
  find_package
- FindFoo files look for ruby and then attempt to require the gem name
  i.e. it checks if the gem can be successfully loaded by the interpreter
- since this is based on find_package it may be influenced in all the
  regular ways (e.g. forced found or disabled from finding altogether
  via CMAKE_DISABLE_FIND_PACKAGE_$PKGNAME)

various notes for the future:
- technically this isn't 100% correct because the require name of a gem and
  the gem name may not be the same. e.g. the gem `docker-api` has the
  require name `docker`. for all currently used gems the names are however
  the same and so simply requiring the gem name is expected to work
- the implementation doesn't care about versions, again because we don't
  need it to
- test-unit is a bundled gem, some distributions (e.g. Arch) do split it
  out without making suitable dependency arrangements on a package level
  though

the tech is heavily inspired by Aleix Pol's tech for finding QML modules
as seen in extra-cmake-modules

Test Plan:
- ruby not found: none of the modules found
- test-unit not present: error
- simplecov not prseent: never errors
- having a module installed or not is reported in the cmake output

Reviewers: #dolphin, elvisangelaccio

Reviewed By: #dolphin, elvisangelaccio

Subscribers: elvisangelaccio, kfm-devel

Tags: #dolphin

Differential Revision: https://phabricator.kde.org/D19856