Support ContextClassLoader when loading optional classes

[AndreasTu]

ReflectionUtil now also uses the ContextClassLoader to load classes, which enables env like OSGi to resolve classes.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 81.48148% with 5 lines in your changes missing coverage. Please review.

Project coverage is 81.89%. Comparing base (2c7db77) to head (23b550a). Report is 127 commits behind head on master.

Files with missing lines	Patch %	Lines
...n/java/org/spockframework/util/ReflectionUtil.java	80.00%	3 Missing and 2 partials :warning:

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #1995      +/-   ##
============================================
+ Coverage     80.44%   81.89%   +1.44%     
- Complexity     4337     4609     +272     
============================================
  Files           441      448       +7     
  Lines         13534    14437     +903     
  Branches       1707     1825     +118     
============================================
+ Hits          10888    11823     +935     
+ Misses         2008     1939      -69     
- Partials        638      675      +37     

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[leonard84]

I think you need to rebase and resolve conflicts first, otherwise my approval gets dismissed anyway.