chore: decouple diagnostics server from collector and handlers

[czeslavo]

What this PR does / why we need it:

Refactors the diagnostics.Server by splitting it into a Collector, Handler, and a Server. That will be needed to enable plugging multiple instances of an HTTPHandler into one mux when we run multiple instances of the manager.

Which issue this PR fixes:

Part of #7042.

[codecov]

Codecov Report

Attention: Patch coverage is 85.54572% with 49 lines in your changes missing coverage. Please review.

Please upload report for BASE (feat/kic-as-library@c317a2b). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
internal/diagnostics/http_handler.go	74.1%	22 Missing and 8 partials ⚠️
internal/diagnostics/collector.go	88.6%	11 Missing and 1 partial ⚠️
internal/manager/run.go	87.5%	5 Missing and 2 partials ⚠️

Additional details and impacted files

@@                  Coverage Diff                  @@
##             feat/kic-as-library   #7079   +/-   ##
=====================================================
  Coverage                       ?   77.0%           
=====================================================
  Files                          ?     216           
  Lines                          ?   24990           
  Branches                       ?       0           
=====================================================
  Hits                           ?   19262           
  Misses                         ?    4730           
  Partials                       ?     998           

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