Create a new type like MinutesSinceEpoch to replace i32 in MetaZonePeriodV1

[samchen61661]

The way to do this would be to make a new type like MinutesSinceEpoch, use it instead of i32 in the ZeroMap, and give it a custom Serialize and Deserialize impl.

Context:
Json file would be more readable, json file API can be customized.
Serialize as raw date timestamp in Json and deserialize as MinutesSinceEpoch.

[sffc]

@samchen61661 Feel free to work on this. If you can get it done by August 10 then we can most likely launch it in ICU4X 1.0.

[sffc]

Discussion:

• It's good to be flexible in JSON parsing, so it's good to support both the current minutes since epoch and the upcoming ISO string, so we don't need to consider this issue a 1.0 data stability blocker
• We should consider this issue blocked on Add Sedate IXDTF (extended ISO) string parsing function #2127