Allow Vision AR SDK client to supply Mapbox Access Token as a parameter

[avi-c]

Right now the iOS SDK requires a mapbox access token to be present in the application's Info.plist file in order to create a VisionManager object

Most apps (including Apex) will want to keep the access token obfuscated somewhere in the code and not present as an easily readable string in the Info.plist file. There should be an exposed constructor for the VisionManager that allows a token to be passed as a parameter.

[stale[bot]]

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