Improvement: Add Distance to saveWorkout, to match workout fetching.

[SeanDunford]

Resolves #357

Background

Save Workout did not include distance which is an odd api choice because fetch workout does include distance. For my own personal needs adding distance to workout saving was necessary and i imagine it will be valuable to other users as well. I did mention a notice of missing permissions somewhere in the code but i tested this code and it works as is. Modeled it after the way calories and intensity are optionally added into the save workout payload.

Mention in comments about permission restriction
https://github.com/StasDoskalenko/react-native-google-fit/blob/master/android/src/main/java/com/reactnative/googlefit/ActivityHistory.java#L390

        // distance scope is missing here due to permission restriction
        // will be added in the future while distance is also available from saveWorkout()

What Changed

Updated WorkoutSample type to include optional distance number `distance?: number

  export type WorkoutSample = {
    startDate: string,
    endDate: string,
    activityType: ActivityType,
    sessionName: string,
    identifier: string,
    description?: string,
    calories?: number,
    steps?: number,
    intensity?: number,
 +    distance?: number,
  }

Conditionally add this as a data set into the session Insert Builder similar to calories and intensity:

if (options.hasKey(DISTANCE_FIELD_NAME)) {
... 
  sessionInsertBuilder.addDataSet(distanceDataSet);
... 
} 

How to test

Submit a workout with distance,
Verify that reading the workout includes the same distance
Go to google fit app and verify it was stored with distance

[SeanDunford]

@StasDoskalenko - not sure if you're still maintaining this project but it would be nice to get this merged. I'm sure you're busy but let me know if you'd like any other code changes before merge.

CC: @aboveyunhai

[StasDoskalenko]

LGTM