[cleanup] shuffle constants in pkg/config

[celenechang]

What does this PR do?

A small cleanup-related change for constants defined in pkg/config . Remove duplicate envvars and move one envvar to apicommon

Also, incorporate DD_DD_URL envvar option into datadog client setup

Motivation

What inspired you to submit this pull request?

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 sure operator builds and runs as expected

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

Attention: Patch coverage is 33.33333% with 4 lines in your changes missing coverage. Please review.

Project coverage is 48.94%. Comparing base (909a576) to head (05379be).

Files with missing lines	Patch %	Lines
pkg/datadogclient/client.go	0.00%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1426      +/-   ##
==========================================
- Coverage   48.94%   48.94%   -0.01%     
==========================================
  Files         221      221              
  Lines       19246    19248       +2     
==========================================
  Hits         9420     9420              
- Misses       9343     9345       +2     
  Partials      483      483              

Flag	Coverage Δ
unittests	48.94% <33.33%> (-0.01%)	⬇️

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

Files with missing lines	Coverage Δ
pkg/config/config.go	67.92% <ø> (ø)
pkg/config/creds.go	100.00% <100.00%> (ø)
pkg/datadogclient/client.go	0.00% <0.00%> (ø)

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