[ios] Add offline event

[zugaldia]

Analogous of #12544 for iOS. We'd like to add a new event to the SDK to help inform offline usage. The Android equivalent is a WIP and it still doesn't support the new region definition introduced by #11447.

/cc: @fabian-guerra @julianrex @rclee @andrlee

[1ec5]

#12544 adds an offline event when creating an offline region on Android. Note that the equivalent operation on iOS would be creating an offline pack; offline regions on iOS correspond to offline region definitions on Android.

[captainbarbosa]

Reopening until #13039 merges as this is also required.

[stale]

This issue has been automatically detected as stale because it has not had recent activity and will be archived. Thank you for your contributions.