Infer main class by searching for class with main method.

[coollog]

If main class is not configured, jib should search for a class with the public static void main method defined and use that class as the main class. If multiple classes are found, an error should be thrown.

This should be as a last-resort, OR if the provided main class is not a valid main class.

This fixes #170

The goal of this is to provide an even more minimal configuration for a developer, where not even a JAR configuration is needed to find the main class.

[coollog]

@TadCordle want to take this?