Add feature flag to opt-into injection per pod

[StefanHauth]

Description

Added feature flag feature.dynatrace.com/automatic-injection to disable automatic injection for a dynakube. If set to false, pods nominated for injection need to be manually annotated with oneagent.dynatrace.com/inject=true. Default value for the feature flag is true.

How can this be tested?

• Set the annotation feature.dynatrace.com/automatic-injection: "false" on a dynakube
• Create a pod or restart one in a namespace that is associated with the used dynakube
• Check that there is no injection happening on the pod.
• Add the annotation oneagent.dynatrace.com/inject: "true" to the pod spec and make sure the pod restarts
• Check that there is injection happening on the pod.

Checklist

• Unit tests have been updated/added
• PR is labeled accordingly
• I have read and understood the contribution guidelines

[meik99]

for the linter, run make go/fmt go/lint to format and lint the file automatically