Allow plugins to override global excludes

[SylvainJuge]

Currently, the agent have a global static list of classes that are excluded from instrumentation.
This list is maintained in co.elastic.apm.agent.bci.ElasticApmAgent#getAgentBuilder method.

Also, there is the internal classes_excluded_from_instrumentation_default configuration that plugins can't modify.

While simple and effective, this implementation has the following disadvantages:

• for internal plugins: adding support for a class that is currently ignored will require to change this exclusion.
  Example : Binary execution support #903 :
• for external plugins: there is no way to bypass the exclusion, hence preventing instrumentation of some classes
  Example : Support for excluding classes from classes_excluded_from_instrumentation_default #1919