Make file time available for dnf plugins

[thebearup]

Makes the "file" time for a given package available for plugin use, to enable virtual snapshots (safe upgrades) based on the time a file was published.

[thebearup]

Context for this pull request: one feature that many users desire is "safe updates." They want to lock into a timestamp at which all updates in a repo have a been tested to be safe/compatible. The Debian ecosystem handles this with repo snapshots, which are cumbersome (an entire copy of the directory structure, which is not very cache-efficient). Since rpm repos have filetime embedded in the repo index, this offers an elegant solution: the client can filter out all packages whose timestamps are greater than the selected timestamp.
From prior investigation, it appears this functionality would best be offered as a dnf plugin vs built-in dnf functionality. However, file time is not exposed to dnf plugins, and build time is a less reliable indicator here (as a package could be built days or weeks ahead of time).
Hence this pull request: expose file time to dnf plugins, so that a plugin can use it for package selection.