Add Metric for Bundle Expiration

[aaronbrethorst]

user story

As a transit manager,
I want to see how many days remain until my bundle expires,
so I can remember to update it

acceptance criteria

• add a static GTFS parsing library: https://github.com/jamespfennell/gtfs
• download the static GTFS once per day and cache it for subsequent metrics reporting
• add a new metrics file/function at internal/metrics/bundle_expiration.go / CheckBundleExpiration(), and wire it up in api/metrics.go
• Push days until bundle expiration into Prometheus (with a gauge status, I think)
• Some tests verifying that this works as expected