chore(refactor): dedicated rels package

[programmer04]

What this PR does / why we need it:

Address comment/fulfill promise

• feat(KongPlugin): generate K8s Events for misconfiguration #6814 (comment)

of putting relations-related code into a separate package. Name rels has been chosen to not be too long and not to steal great variable names like relation, relations and rel.

[codecov]

Codecov Report

Attention: Patch coverage is 80.76923% with 5 lines in your changes missing coverage. Please review.

Project coverage is 76.4%. Comparing base (b14df00) to head (5504b84).
Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
internal/dataplane/kongstate/kongstate.go	82.3%	3 Missing ⚠️
internal/dataplane/kongstate/customentity.go	77.7%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##            main   #6889      +/-   ##
========================================
+ Coverage   59.0%   76.4%   +17.3%     
========================================
  Files        210     212       +2     
  Lines      25241   25246       +5     
========================================
+ Hits       14917   19297    +4380     
+ Misses      9030    4928    -4102     
+ Partials    1294    1021     -273     

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