Support retrieving timezone metadata via properties other than IANA code

Environment

• Package version(s): datetime2 v0.9.6

Feature request

@blueprintjs/datetime2 supports getTimezoneMetadata() where you pass in a timezone IANA code. But sometimes I'd like to query for the metadata with other information, like the UTC offset.

When querying by only the offset, we know that one offset may correspond to multiple timezones. In these cases we may only care about the minimal set of timezones (see https://github.com/palantir/blueprint/issues/5793).

We could probably support this in the existing API:

function getTimezoneMetadata(offsetOrIanaCode: string, date?: Date): TimezoneMetadata | undefined;