Add a flag to enable/disable DatadogAgent controller

[levan-m]

What does this PR do?

Adding a flag to control creation of DatadogAgent controller (and webhook), similar to what we have DatadogMonitor here.

Helm change an doc update to follow.

Motivation

#721

Additional Notes

Anything else we should know when reviewing?

Describe your test plan

• Set flag to false, build and deploy Operator.
• Delete DatadogAgent CRD if exists.
• Watch for any Operator errors.
• Apply DatadogMonitor manifest, confirm monitor is created.

[codecov-commenter]

Codecov Report

Merging #731 (5c98b1c) into main (411a462) will increase coverage by 0.03%.
The diff coverage is 84.37%.

@@            Coverage Diff             @@
##             main     #731      +/-   ##
==========================================
+ Coverage   58.75%   58.79%   +0.03%     
==========================================
  Files         149      149              
  Lines       17814    17831      +17     
==========================================
+ Hits        10467    10484      +17     
- Misses       6727     6728       +1     
+ Partials      620      619       -1     

Flag	Coverage Δ
unittests	58.79% <84.37%> (+0.03%)	⬆️

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

Impacted Files	Coverage Δ
controllers/setup.go	56.52% <0.00%> (-4.42%)	⬇️
...ontrollers/datadogagent/merger/security_context.go	89.18% <100.00%> (+25.18%)	⬆️

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