feat: combine services across different httproutes for traditional routes

[randmonkey]

What this PR does / why we need it:

Combine rules sharing the same backends cross HTTPRoutes in the same namespace into a single Kong gateway service.
This PR resolves the case for traditional routes (traditional or traditional_compatible router flavor).

Which issue this PR fixes:

fixes #6726
Special notes for your reviewer:

Moved part of code for translating HTTPRoute to the package subtranslator.
GenerateKongRouteFromTranslation is used by the validation server so this function and its dependencies are not touched.
backendref.go and backendref_test.go are copied into subtranslator package since the original one is still used by GRPCRoute translator. Will process them in the later PR.

PR Readiness Checklist:

Complete these before marking the PR as ready to review:

• the CHANGELOG.md release notes have been updated to reflect any significant (and particularly user-facing) changes introduced by this PR

[codecov]

Codecov Report

Attention: Patch coverage is 82.14936% with 98 lines in your changes missing coverage. Please review.

Project coverage is 78.2%. Comparing base (f187f31) to head (ffa3bbf).
Report is 8 commits behind head on main.

Files with missing lines	Patch %	Lines
...al/dataplane/translator/subtranslator/httproute.go	81.3%	66 Missing and 14 partials ⚠️
...l/dataplane/translator/subtranslator/backendref.go	84.6%	9 Missing and 3 partials ⚠️
...ternal/dataplane/translator/translate_httproute.go	76.9%	5 Missing and 1 partial ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #6711     +/-   ##
=======================================
+ Coverage   77.8%   78.2%   +0.3%     
=======================================
  Files        204     205      +1     
  Lines      23870   24238    +368     
=======================================
+ Hits       18590   18965    +375     
+ Misses      4337    4318     -19     
- Partials     943     955     +12     

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