Fix DatadogMetric CRD: references should be optional (will be filled by DCA 1.9+)

[vboulineau]

What does this PR do?

Make autoscalerReferences optional as it's not filled by DCA ATM (will require DCA 1.9+)

Motivation

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Additional Notes

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[codecov-commenter]

Codecov Report

Merging #130 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #130   +/-   ##
=======================================
  Coverage   58.43%   58.43%           
=======================================
  Files          31       31           
  Lines        4376     4376           
=======================================
  Hits         2557     2557           
  Misses       1630     1630           
  Partials      189      189           

Flag	Coverage Δ
#unittests	58.43% <ø> (ø)

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