add DatadogAgent v2alpha1 defaulting code

[celenechang]

What does this PR do?

Add defaulting code for DatadogAgent v2alpha1 CRD

Motivation

Additional Notes

Currently the code is not called in the reconcile loop

Describe your test plan

[kayayarai]

Minor edits

[vboulineau]

I wonder if we should default false default value? I am saying that because it adds an unnecessary burden to anyone adding a new feature (to keep consistent code and having a mixup of features that are defaulted to false and not some others).

Also wondering if this should not be the place where the image, registry, etc. is defaulted?

[codecov-commenter]

Codecov Report

Merging #477 (472e6eb) into main (3fe793b) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #477   +/-   ##
=======================================
  Coverage   59.68%   59.68%           
=======================================
  Files           3        3           
  Lines         129      129           
=======================================
  Hits           77       77           
  Misses         40       40           
  Partials       12       12           

Flag	Coverage Δ
unittests	59.68% <ø> (ø)

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