chore: don't use KonnectClient in ClientsProvider

[czeslavo]

What this PR does / why we need it:

Stop using KonnectClient in the clients' provider. Make konnect.ConfigSynchronizer create one using a factory on its startup instead. Also:

• moves tests related to Konnect-specific behavior to konnect.ConfigSynchronizer.
• moves call to deckgen.ToDeckContent from KongClient to konnect.ConfigSynchronizer as it may reach out to Konnect APIs while for plugins schemas, and we don't want to spend time on Konnect-related stuff in the critical path

Which issue this PR fixes:

Part of #6340.

[codecov]

Codecov Report

Attention: Patch coverage is 10.34483% with 26 lines in your changes missing coverage. Please review.

Project coverage is 4.0%. Comparing base (b14df00) to head (592d5dd).
Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
internal/adminapi/konnect.go	0.0%	14 Missing ⚠️
internal/dataplane/sendconfig/dbmode.go	30.0%	6 Missing and 1 partial ⚠️
internal/util/clock/ticker.go	0.0%	4 Missing ⚠️
internal/dataplane/sendconfig/strategy.go	0.0%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##            main   #6885      +/-   ##
========================================
- Coverage   59.0%    4.0%   -55.1%     
========================================
  Files        210     138      -72     
  Lines      25241   15141   -10100     
========================================
- Hits       14917     606   -14311     
- Misses      9030   14465    +5435     
+ Partials    1294      70    -1224     

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