Add way to have utils.data functions w/ cache have their cache expire

Right now, the various functions in `utils.data` that have a `cache` option hold onto their cached copy of a download until the file is deleted. There are some use cases that require the cache expire after a certain time, and get re-downloaded (e.g. #1145).

So these functions (which all depend on `download_file` I believe), should allow `cache` to give a specific timeframe after which the file should be re-downloaded. This is pretty straightforward, but there is a question how best to define the expiration time. My instinct is to use stdlib `datetime.datetime` and `datetime.timedelta` objects, but an alternative might be `astropy.time.Time` and `astropy.time.DeltaTime`.

Start by tracing the cache-enabled functions in utils.data and their shared download_file dependency. Decide how the cache option should represent an expiration interval, considering the datetime/datetime.timedelta and astropy.time alternatives raised in the issue. Done means cached downloads are refreshed after the configured interval while existing cache behavior remains supported.

Written by the indexing model from the issue text.