Support retrieval of the host system time zone as icu_timezone::CustomTimeZone

[hsivonen]

Instantiating an icu_timezone::CustomTimeZone that represents the current time zone of the host system is an operation that is likely to be commonly needed by apps while requiring special expertise (as well as access to alias data) to implement correctly. Therefore, it would be natural for ICU4X to provide this operation.

AFAICT, ICU4C provides this but both Chromium and Firefox work around https://unicode-org.atlassian.net/browse/ICU-13694 , so ICU4X should be written not to need a workaround.

(Since this operation is inherently both operating system-dependent and operating system-specific, it obviously can't be an inseparable part of icu_timezone but either needs to be omissible using cargo features or needs to be in its own crate so that icu_timezone continues to work in a mere core+alloc context.)