Add Support for Firo View Keys

[cassandras-lies]

This PR adds support for Firo View Keys. It also contains a commit to add iOS simulator support, and two commits which SHOULD NOT be merged, and should be replaced with the cypherstack version of flutter_libsparkmobile after it has merged https://github.com/cypherstack/flutter_libsparkmobile/pull/45 Additionally, parts of one commit that should not be merged contains modifications that should be merged that allow building flutter_libsparkmobile on iOS simulator.