Readds separate proxy key to the init secret

[0sewa0]

Description

The proxy field in the init secret was moved under the config field (which is a json).
This broke how the webhook injects the DT_PROXY envvar to the customer's pod.

And to avoid the need to directly read the secret in the webhook during injection, this adds the proxy field back as its separate field.
This duplicates the proxy value inside the init secret, however this is a far nicer way than reading the secret for every pod injection.

How can this be tested?

Use a dynakube that has proxy set, and you can check:

• the dynatrace-dynakube-config in the sample app namespace has the proxy field
• exec into the sample pod, check if DT_PROXY is set
• everything works as normal

Checklist

• Unit tests have been updated/added
• PR is labeled accordingly