Provide computation of UTC offset given an epoch moment and a named time zone

[hsivonen]

Temporal requires the operation GetNamedTimeZoneOffsetNanoseconds, which takes an epoch moment and a named time zone and returns the UTC offset of the named time zone on the given moment.

For Temporal compat, ICU4X should provide an API for this.

[sffc]

CC @leftmostcat @nordzilla