feat: add market_schedule module

[keyvankhademi]

This add marker_schedule module and renames the old schedule to legacy_schedule.
It uses a new metadata key schedule and it is backward compatible by falling back to weekly_schedule metadata key.

[Reisen]

I might be missing context, but is the string format for these holiday schedules defined anywhere or is it being introduced by this PR? I think it would be worth using a parsing library here to make the parsing a bit more robust. While FromStr is the right place to put this code I think handling parsing more robustly via something like winnow might be the right thing to do to define this format more concretely.

[ali-bahjati]

Very nice!