Update the Hawkbit chart to be compatible with external secrets

[joshua-clayton]

I found that the Hawkbit chart was not compatible with 'existingSecret', and also that I couldn't even inject a secret via extraEnv,

I ended up replacing SPRING_APPLICATION_JSON with several individual environment variables, in order to have more fine grained control. Where possible I've kept it backwards compatible, though I'm not sure if that is the best way forward, I either need to do more work to be perfectly backward compatible, or provide a migration guide, and explain how the values.yaml needs to change to move to this version.

I'm just. starting to test this updated chart, so there may be bugs. I also hope to update to the latest version of hawkbit, but my first attempt to do that didn't go well.

It is not my desire to permanently fork the chart, I'm pushing this as a draft PR to see if I get any feedback. I'm happy to make changes to better suit the project.