Add DDM service struct, basic handlers, and test client

[gillespi314]

No description provided.

[codecov]

Codecov Report

Attention: Patch coverage is 70.00000% with 18 lines in your changes are missing coverage. Please review.

❗ No coverage uploaded for pull request base (feat-macos-ddm@d261762). Click here to learn what that means.

Files	Patch %	Lines
server/service/apple_mdm.go	65.85%	12 Missing and 2 partials ⚠️
cmd/fleet/serve.go	0.00%	3 Missing ⚠️
server/service/handler.go	50.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@                Coverage Diff                @@
##             feat-macos-ddm   #17671   +/-   ##
=================================================
  Coverage                  ?   65.66%           
=================================================
  Files                     ?     1194           
  Lines                     ?   108235           
  Branches                  ?     2574           
=================================================
  Hits                      ?    71071           
  Misses                    ?    31770           
  Partials                  ?     5394           

Flag	Coverage Δ
backend	66.66% <70.00%> (?)

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