Add support for specifying multiple processor classes on a java_annotation_processor rule

[raviagarwal7]

Single annotation processors can have multiple processor classes. for eg: AutoValue has these

'com.google.auto.value.processor.AutoAnnotationProcessor',
'com.google.auto.value.processor.AutoValueBuilderProcessor',
'com.google.auto.value.extension.memoized.MemoizedValidator',
'com.google.auto.value.processor.AutoValueProcessor',

This now requires creating multiple java_annotation_processor rule.

Would be great if we can specify multiple processor classes. propsBuilder already supports adding multiple processor classes.

Proposed Solution: Add a new argument to java_annotation_processor processor_classes so it is backward compatible and processor_class can be removed afterwards.

[raviagarwal7]

cc: @jkeljo, @kageiit