Provide API/documentation on how to allow/disallow blocking calls inside static initializer

[simonbasle]

See also #174, where a similar API/documentation should be provided for constructors.

Quoting from the above:

There is no guidance I could find on how to allow/disallow blocking in a constructor or static initializer. There is one test that shows how to do it for a static initializer but it would be nicer if it was easier to find. One option could be a dedicated method, e.g. allowBlockingCallsInsideConstructor(String classname), or otherwise provide guidance in the Javadoc on allowBlockingCallsInside and a mention on the Customizations page.

@rstoyanchev you suggested that additional methods would maybe increase the API too much, and suggested enums or varargs. How about a couple constants? Something like:

//in Blockhound.Builder class, or directly at root Blockhound.java
public static final String STATIC_INITIALIZER = "<clinit>";

//in user code
BlockHound.install(b -> b.allowBlockingCallsInside(ClassWithStaticInit.class.getName(), BlockHound.Builder.STATIC_INITIALIZER));

[simonbasle]

going with the minimum viable solution of directly documenting <clinit> in the methods javadocs