Add hostIP var to operator

[buckner]

Hello!

I'm running DataDog agents as a Daemonset and they are capable of collecting OTEL metrics, however I need the .Values.opentelemetry.collector.uri variable to refer to the node's IP address. If I use .Values.env to add the hostIP downward api variable it is lower in the env list and thus lower in precedence than .Values.opentelemetry.collector.uri. Unfortunately it doesn't get replaced when I set it like this:

- name: OTEL_EXPORTER_OTLP_ENDPOINT
  value: http://$(HOST_IP):4318

When I put the env var near the top of the list $(HOST_IP) gets substituted correctly.

Relevant DataDog documentation:

I'm happy to put this behind a value for toggling if that's preferred.

Checklist

• [x] I have verified that my change is according to the deprecations & breaking changes policy
• [x] Commits are signed with Developer Certificate of Origin (DCO - learn more)
• [x] README is updated with new configuration values (if applicable) learn more
• [x] A PR is opened to update KEDA core (repo) (if applicable, ie. when deployment manifests are modified)

Fixes #