TestApp: Make Certificate selectable from device

[wba2hi]

Is your feature request related to a problem? Please describe.
Right now we use a hard coded custom certificate. Therefore the DataBroker always needs to be started with the same certificate as well. We should add more flexibility by allowing the user to select a custom certificate used for encrypted traffic.

Describe the solution you'd like
Instead of using a hard-coded custom certificate it should be possible to select a custom certificate to use for authentication. Error management is important, if an invalid file is selected or no certificate at all, the error management should be appropriate, the app should not crash.