…instrumentation (elastic#124404)

Using ASM CheckClassAdapter was key to diagnose the issue we had with incorrect signatures for some check methods.
In this PR I polished up the code I used to pinpoint the issue, and made it available via a system property so it can be turned on if we need it (and it's always on for Entitlements IT tests too).

It is also turned on in case we get VerifyErrors during retransformClasses early in the Entitlement agent bootstrap: retransformClasses runs in the native part of the JVM, so the VerifyError it produces is not so readable (e.g. it lacks a full stack trace and a description); in case this happens, we re-apply the transformation with verification turned on to get a meaningful error before dying.