feat(injector): allow configuring injector version

[betterengineering]

What does this PR do?

This commit allows users to optionally pin the injector version to a specific image tag for the Single Step APM Instrumentation in Kubernetes.

Motivation

Both internally during development of SSI and externally for our customers, it's useful to be able to pin the injector version. We already support this in the datadog Helm chart and this expands the functionality for customers using the operator.

Additional Notes

This is my first change to this repo! Let me know if I missed anything.

Minimum Agent Versions

• Cluster Agent: 7.57.0+

Describe your test plan

I setup minikube locally with a registry. I ran the following to install my operator based on the docs:

# Build
make IMG=localhost:5000/test/operator:test IMG_CHECK=localhost:5000/test/operator-check:test docker-build

# Push
make IMG=localhost:5000/test/operator:test IMG_CHECK=localhost:5000/test/operator-check:test docker-push

# Deploy
make IMG=localhost:5000/test/operator:test IMG_CHECK=localhost:5000/test/operator-check:test deploy

I then added the following config to examples/datadogagent/datadog-agent-minimum.yaml and ran the script for a DatadogAgent resource:

  features:
    apm:
      instrumentation:
        enabled: true
        injector:
          imageTag: "0.25.0"
        enabledNamespaces:
          - "test-1"
        libVersions:
          java: "1"
          dotnet: "3"
          python: "2"
          js: "5"

Finally, I deployed an example JS app to the test-1 namespace and ensured it got the old injector version:

gcr.io/datadoghq/apm-inject:0.25.0

Checklist

• PR has at least one valid label: bug, enhancement, refactoring, documentation, tooling, and/or dependencies
• PR has a milestone or the qa/skip-qa label

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 49.33%. Comparing base (d49f7c2) to head (d8e3892).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1614      +/-   ##
==========================================
+ Coverage   49.28%   49.33%   +0.04%     
==========================================
  Files         216      216              
  Lines       20662    20680      +18     
==========================================
+ Hits        10184    10202      +18     
  Misses       9942     9942              
  Partials      536      536              

Flag	Coverage Δ
unittests	49.33% <100.00%> (+0.04%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
...ller/datadogagent/defaults/datadogagent_default.go	91.17% <100.00%> (+0.07%)	⬆️
...nal/controller/datadogagent/feature/apm/feature.go	65.51% <100.00%> (+1.48%)	⬆️
pkg/testutils/builder.go	91.56% <100.00%> (+0.03%)	⬆️

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