Add FileContents to AuditEvent

Currently, `FileContents` is created in each `AbstractFileSetCheck` descendant. This is redundant, since the file should not change and the result is always the same. This should be done once in the `Checker.processFiles` method. By doing this, we not only speed up the process, but also provide access to the file contents for `AuditListeners`.

_Originally posted by @pbludov in https://github.com/checkstyle/checkstyle/issues/8807#issuecomment-723415882_