[ICD] Darwin implementation for the ICD Storage

[yunhanw-google]

#29783 this PR does not provide ICDStorage per identify, per @bzbarsky-apple , darwin needs to implemented the dedicated per-identity storage for ICD.

[bzbarsky-apple]

This should get piped through the existing machinery for these things, ideally.

[yunhanw-google]

yes, agree

[yunhanw-google]

just update the ticket description, thanks

[bzbarsky-apple]

It's really not clear so far based on the API definitions in the core which parts this implementation will have to do. This needs to wait until either the core part calling into the API is done or until the API has better documentation. Ideally the latter. @yunhanw-google @tcarmelveilleux