Motivation:
Classes might be initialized inside the event loop where blocking calls are otherwise forbidden.
The JdkSslContext has a blocking call in its class initializer through initializing the Java SSLContext which does IO to deserialize its key store.

Modification:
Collect the class initialization code in a method, and allow it to have blocking calls in blockhound.

Result:
No more blockhound warnings about blocking calls in JdkSslContext class initializer.

Fixes netty#12425