Parse IANA files into ICU4X data formats for TzdbDataProvider

[nordzilla]

One of the formats we should be able to generate ICU4X TimeZone data from is the human-readable file format provided by the IANA TZDB

There are two ways we can go about this:

1. Build the zic C binary included in the IANA TZDB, and use it to generate TZif files. Then use our TZif capabilities from TODO(NUMBER) to generate the ICU4X data.
2. Write a custom parser to go directly from IANA files to ICU4X data.

This will then match the combined --tz-src-format iana and --tz-src-path options in the datagen crate (see TODO(Update datagen crate ... from file system))

This will also match the --tz-src-version options in the datagen crate.

For more context see

• #1009

Depends on:

• #998
• #999

[sffc]

@nordzilla Please assign this to an appropriate 0.6 sprint milestone.