FileMetadata with last modification information

[claudiofelber]

Please add file timestamp information to the FileMetadata class. okio's FileMetadata provides the following three properties:

val createdAtMillis: Long
val lastModifiedAtMillis: Long
val lastAccessedAtMillis: Long

This or something comparable would be really helpful. File timestamp information is required for a lot of use cases, such as determining whether a cached file needs to be updated or not.