support parsing individual opening dates

[MoralCode]

the schema from vaccine-feed-ingest supports this:

opening_dates: [
    {
      opens: str as date that facility opens in iso8601 format e.g. 2021-04-15,
      closes: str  as date that facility closes in iso8601 format e.g. 2021-06-15,
    },
    ...
  ],
  opening_hours: [
    {
      day: str as lowercase day of week e.g. monday,
      opens: str as time with facility opens on this day in format hh:mm,
      closes: str as time with facility closes on this day in format hh:mm,
    },```