Subtask 1: feat: Add option to select which metric to be displayed in the notification bar

[ShivamSoni24]

Is your feature request related to a problem? Please describe.

The current notification in OpenTracks lacks useful information for users who are not interested in speed competition but are concerned about their heart rate due to health issues. The notification primarily displays speed, which is not relevant to all users. For individuals with heart problems, having quick access to real-time heart rate information is crucial. Therefore, there is a need to customize the notification content or, at the very least, have an option to display heart rate instead of speed.

Describe the solution you'd like

I would like the ability to customize the content displayed in the OpenTracks notification, allowing users to choose which parameter is most relevant to them—whether it be speed, heart rate, distance. Specifically, I am requesting the option to replace the speed display in the notification with the current heart rate. This customization would enhance the user experience by providing personally relevant information at a glance.

Describe alternatives you've considered

One alternative could be a toggle option within the app settings that allows users to choose between displaying speed or heart rate in the notification. This way, users with different preferences and priorities can easily customize their experience without having to navigate through multiple settings.

Additional context

Having the option to see heart rate in the notification would greatly enhance the app's utility for users with health concerns. I believe this customization feature would make OpenTracks more inclusive and adaptable to a wider range of user needs.

Original Issue #177