Load environment variables in override with envFrom

[swang392]

What does this PR do?

Add ability for environment variables to be loaded in the override map of datadog-agent.yaml using envFrom.
Example:

Motivation

#896

Additional Notes

Anything else we should know when reviewing?

Minimum Agent Versions

Are there minimum versions of the Datadog Agent and/or Cluster Agent required?

• Agent: vX.Y.Z
• Cluster Agent: vX.Y.Z

Describe your test plan

• make a kind cluster with Operator
• Make and load some test configMaps and secrets into kind cluster. To double check the priority between env and envFrom (env should take priority), add a field DD_CLUSTER_NAME: <not your cluster name>
• Add fields into datadog-agent.yaml: spec.override.nodeAgent.EnvFrom, spec.override.clusterAgent.envFrom, and/or spec.override.clusterChecksRunner.envFrom with configMaps or secrets
• add Agent to kind cluster
• run kubectl describe pod for agent, cluster checks runner, and cluster agent and verify that the configs or secrets that you specified are there. They should be in a separate section right above the environment variables.
• run kubectl exec -it <datadog agent pod> bash and then env. You should be able to see the specific variables you added with their values.
• For agent that you modified DD_CLUSTER_NAME for, when you run the exec command, the DD_CLUSTER_NAME field should not be overridden to what was put in the configMap/secret.

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

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[codecov-commenter]

Codecov Report

Attention: Patch coverage is 11.70213% with 83 lines in your changes missing coverage. Please review.

Project coverage is 48.75%. Comparing base (fdaeb06) to head (eecc545).

Files with missing lines	Patch %	Lines
...ler/datadogagent/merger/fake/envfromvar_manager.go	10.86%	41 Missing ⚠️
...ernal/controller/datadogagent/merger/envvarfrom.go	13.15%	32 Missing and 1 partial ⚠️
...ontroller/datadogagent/override/podtemplatespec.go	0.00%	3 Missing and 1 partial ⚠️
internal/controller/datadogagent/feature/types.go	0.00%	3 Missing ⚠️
...r/datadogagent/feature/fake/PodTemplateManagers.go	33.33%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1429      +/-   ##
==========================================
- Coverage   48.95%   48.75%   -0.20%     
==========================================
  Files         221      222       +1     
  Lines       19252    19342      +90     
==========================================
+ Hits         9424     9431       +7     
- Misses       9345     9425      +80     
- Partials      483      486       +3     

Flag	Coverage Δ
unittests	48.75% <11.70%> (-0.20%)	⬇️

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

Files with missing lines	Coverage Δ
api/datadoghq/v2alpha1/datadogagent_types.go	100.00% <ø> (ø)
...r/datadogagent/feature/fake/PodTemplateManagers.go	92.59% <33.33%> (-7.41%)	⬇️
internal/controller/datadogagent/feature/types.go	26.16% <0.00%> (-0.76%)	⬇️
...ontroller/datadogagent/override/podtemplatespec.go	71.76% <0.00%> (-0.97%)	⬇️
...ernal/controller/datadogagent/merger/envvarfrom.go	42.85% <13.15%> (-57.15%)	⬇️
...ler/datadogagent/merger/fake/envfromvar_manager.go	10.86% <10.86%> (ø)

... and 1 file with indirect coverage changes

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[fanny-jiang]

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