com.google.cloud.tools.jib.plugins.common.PluginConfigurationProcessor#createModificationTimeProvider does not support USE_CURRENT_TIMESTAMP

Hi,

I thought the issue was already created but github didnt popped it up so creating this issue hoping it is not a duplicated.
Long story short, there are numerous cases (think serving static content for ex) where we want to get the build time as file time for extra dir and today it requires build hacks whereas `USE_CURRENT_TIMESTAMP` is a neat way for the classpath/classes, would be great to add it in the switch for `createModificationTimeProvider` too if possible.