Add IDefaultValueProviderExtension

[leonard84]

to make EmptyOrDummyResponse extensible for type that need special handling.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 90.90909% with 2 lines in your changes missing coverage. Please review.

Project coverage is 81.88%. Comparing base (2c7db77) to head (99fc3a8). Report is 119 commits behind head on master.

Files with missing lines	Patch %	Lines
.../org/spockframework/mock/EmptyOrDummyResponse.java	90.90%	1 Missing and 1 partial :warning:

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #1994      +/-   ##
============================================
+ Coverage     80.44%   81.88%   +1.43%     
- Complexity     4337     4603     +266     
============================================
  Files           441      448       +7     
  Lines         13534    14394     +860     
  Branches       1707     1822     +115     
============================================
+ Hits          10888    11787     +899     
+ Misses         2008     1936      -72     
- Partials        638      671      +33     

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[leonard84]

@AndreasTu thanks for the review, I've addressed the comments