Let user customize their walking speed

[aaronbrethorst]

The user should be able to customize their speed of movement, which is used to determine how quickly they can reach their transit stop.

The user should be able to customize this value in two ways:

1. By typing a new number into a text box in Settings.

2. By retrieving this data from HealthKit in iOS 14

HealthKit in iOS 14 adds support for determining and accessing the user's walking speed. There should be an option for retrieving this data from the HealthKit store instead of simply using the hardcoded value of 3.1 miles/hour I've included.

https://developer.apple.com/videos/play/wwdc2020/10182/