This PR refactors the Swift native implementation. It introduces the following new classes:

• HealthDataWriter: writing health data to HealthKit functions
• HealthDataReader: reading health data from HealthKit functions
• HealthDataOperations: managing data permissions and delete functions
• HealthConstants: constants used in the plugin (where types are introduced)
• HealthUtilities: helper functions - also includes type conversion helpers for HKWorkoutActivityType

The initialisation of these classes are lazy now to help with performance.

In the new approach, when defining a new method for iOS native, for better error handling the cases should be handled as follow under the handle(_ call: FlutterMethodCall, result: @escaping FlutterResult) function:

case "writeMeal":
   do {
         try healthDataWriter.writeMeal(call: call, result: result)
    } catch {
         result(FlutterError(code: "WRITE_ERROR",
                    message: "Error writing meal: \(error.localizedDescription)",
                    details: nil))
    }

This now requires each new method to throw an error in case of failure.

Supported error codes to be used:

• REQUEST_AUTH_ERROR: Error requesting authorisation
• PERMISSION_ERROR: Error checking permissions
• WRITE_ERROR: Error writing [DataType]
• DELETE_ERROR: Error deleting data
• ARGUMENT_ERROR: Missing required argument
• INVALID_TYPE: Invalid dataTypeKey
• HEALTH_ERROR: Error getting health data
• INTERNAL_ERROR: Internal instance reference lost
• STATISTICS_ERROR: Error getting statistics
• STEPS_ERROR: Error getting step count