Fix Illegal Access Error

[kanderson250]

Resolves #2290

This PR resolves the Illegal Access Error that has plagued environments using the agent with Scala 2.12 and JDK 11.

The fix is to avoid bumping class file versions to the runtime version, as the agent does normally, for apps that have Scala 2.12 in the environment. Since Scala 2.12 is not always detectable by the agent (we check the system classloader for this - notably, sbt will load Scala classes into custom Scala loaders), there is also a feature flag to manually enable the fix via system property -Dnewrelic.config.class_transformer.illegal_access_fix=true.

I added an AIT for the IllegalAccessError. The branch for that is here with AITs run results here.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 46.66667% with 8 lines in your changes missing coverage. Please review.

Project coverage is 70.48%. Comparing base (a0d08e3) to head (17766ec).

Files with missing lines	Patch %	Lines
...main/java/com/newrelic/weave/utils/WeaveUtils.java	42.85%	6 Missing and 2 partials ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #2334      +/-   ##
============================================
- Coverage     70.58%   70.48%   -0.10%     
+ Complexity    10042    10025      -17     
============================================
  Files           844      844              
  Lines         40750    40627     -123     
  Branches       6191     6160      -31     
============================================
- Hits          28762    28637     -125     
+ Misses         9210     9201       -9     
- Partials       2778     2789      +11     

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