Workaround for big numbers in JSONs

This MR introduces a workaround for an issue where JSON models containing big numbers (eg 930-digits) can't be parsed by swift's JSONDecoder/JSONEncoder.

This affects the GroupMember model, which holds DHKey models that has value property with such a big number. The model's value is stored as String, and it's automatically decoded/encoded to big number in JSON.

requested review from @brunomunizaf

assigned to @darrarski.xx

mentioned in commit f2a547dd

merged