feat: add the ability to instantiate GreenMail per-class in addition to per-method

[netmikey]

I had a use case where I needed GreenMail to be instantiated once per test class instead of once per test method. This behaviour was added to the JUnit5 GreenMailExtension. Of course, this change is made backwards compatible so it doesn't affect existing code. The corresponding test class com.icegreen.greenmail.junit5.AlternateLifecyclesTests shows how the lifecycle can be switched, essentially:

@RegisterExtension
GreenMailExtension greenMail = new GreenMailExtension()
    .withPerMethodLifecycle(false)
    .withConfiguration(...);

The extension works very well, please consider publishing the JUnit5 module to Maven Central.

[marcelmay]

Thx alot, @netmikey !