Classpath attribute ignore_optional_problems should be supported

[laeubi]

<classpathentry kind="src" path=src-gen/">
        <attributes>
                <attribute name="ignore_optional_problems" value="true"/>
        </attributes>
</classpathentry>

should result in something similar to:

<compilerArgument>-nowarn:[${basedir}/src-gen]</compilerArgument>

See

• https://help.eclipse.org/latest/topic/org.eclipse.jdt.doc.user/tasks/task-using_batch_compiler.htm
• https://github.com/eclipse-jdt/eclipse.jdt.core/discussions/2239