Chore: Define dedicated types for DBMode and RouterFlavor

[randmonkey]

What this PR does / why we need it:

Define dedicated types for DB mode and router flavor in util/dataplane for clarity and prevent definitions of constants everywhere.
Which issue this PR fixes:

fixes #5013

Special notes for your reviewer:

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

[programmer04]

Check also

• test/internal/testenv/testenv.go
• internal/dataplane/synchronizer_test.go:25 (and more)
• internal/clients/manager_test.go:228
• internal/manager/utils/kongconfig/root_test.go
• internal/dataplane/kong_client_test.go:684

I might miss something

[codecov]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (27c29b4) 75.6% compared to head (f0a811c) 75.5%.
Report is 3 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #5092     +/-   ##
=======================================
- Coverage   75.6%   75.5%   -0.1%     
=======================================
  Files        167     168      +1     
  Lines      18906   18905      -1     
=======================================
- Hits       14295   14288      -7     
- Misses      3787    3791      +4     
- Partials     824     826      +2     

Files	Coverage Δ
internal/clients/manager.go	96.2% <100.0%> (ø)
internal/dataplane/config/router.go	100.0% <100.0%> (ø)
internal/dataplane/kong_client.go	88.9% <100.0%> (ø)
internal/dataplane/synchronizer.go	93.5% <100.0%> (ø)
internal/dataplane/translator/translator.go	89.7% <100.0%> (-2.0%)	⬇️
internal/manager/run.go	55.4% <100.0%> (-0.2%)	⬇️
internal/manager/setup.go	77.1% <100.0%> (ø)
internal/manager/utils/kongconfig/root.go	51.3% <100.0%> (+0.6%)	⬆️
internal/dataplane/config/dbmode.go	90.9% <90.9%> (ø)

... and 1 file with indirect coverage changes

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