tracing-appender: Introduce weekly rotation

[NickCaplinger]

Motivation

While configuring tracing-appender, I wanted to specify a weekly log rotation interval. I was unable to do so, as the largest rotation interval was daily.

Solution

Before my introduction of weekly log rotation, rounding the current OffsetDateTime was straightforward: we could simply keep the current date and truncate part or all of the time component. However, we cannot simply truncate the time with weekly rotation; the date must now be modified.

To round the date, I take its ordinal (how many days have passed since the start of the year) and round it backwards to the nearest 7 day period. This gives us consistent date-times that only change weekly.