libcharon: Add a test runner
attributes: Move the configuration attributes framework to libcharon
unit-tester: Drop the old unit-tester libcharon plugin

While it has some tests that we don't directly cover with the new unit tests,
most of them require special infrastructure and therefore have not been used
for a long time.
attribute-manager: Pass the full IKE_SA to provider methods
attribute-manager: Pass full IKE_SA to handler methods
attribute-provider: Pass full IKE_SA to provider backends
attribute-handler: Pass full IKE_SA to handler backends