Parse ICU files into ICU4X data formats

[nordzilla]

One of the formats from which we should be able to generate ICU4X TimeZone data is the ICU format, particularly in the zoneInfo64.txt files.

We will have to write our own parser for this.

This will then match the combined --tz-src-format icu and --tz-src-path in the datagen crate.

For more context see

• Update datagen crate to generate IANA TZDB data from file system #1008

Depends on:

• Create an initial tzdb-provider crate #998
• Design ICU4X ZoneInfo data structs for TZDB data #999

[sffc]

I think this is more of an enhancement right now than a core feature. I'll change the label. (Same with #1002)