feat(konnect): add client configuration

[czeslavo]

What this PR does / why we need it:

• Adds Konnect flags (default-false --konnect-sync-enabled feature flag along with Konnect Admin API client-specific configuration)
• Adds a constructor for Konnect Admin API (NewKongClientForKonnectRuntimeGroup) that is to be used in the next PRs
• Refactors code around TLS client certificates to not duplicate code handling regular Admin API and Konnect configs

Which issue this PR fixes:

Part of #3437.

Notes to reviewers:

Even though the PR adds Konnect Admin Admin API client implementation, it does not use it yet. Next PRs are going to bring more tests and integrate the Konnect client into the dataplane loop. It's to keep the size of the PRs reasonable.

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

Base: 73.8% // Head: 73.6% // Decreases project coverage by -0.3% ⚠️

Coverage data is based on head (a1f7141) compared to base (de3499f).
Patch coverage: 56.9% of modified lines in pull request are covered.

❗ Current head a1f7141 differs from pull request most recent head 3c1e696. Consider uploading reports for the commit 3c1e696 to get more accurate results

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #3455     +/-   ##
=======================================
- Coverage   73.8%   73.6%   -0.3%     
=======================================
  Files        115     118      +3     
  Lines      13971   14067     +96     
=======================================
+ Hits       10322   10356     +34     
- Misses      2993    3051     +58     
- Partials     656     660      +4     

Impacted Files	Coverage Δ
internal/adminapi/konnect.go	0.0% <0.0%> (ø)
internal/cmd/rootcmd/run.go	21.0% <0.0%> (ø)
internal/dataplane/kong_client.go	87.1% <ø> (ø)
internal/manager/utils/kongconfig/root.go	49.5% <ø> (ø)
internal/adminapi/client.go	13.3% <15.3%> (-42.5%)	⬇️
internal/adminapi/tls.go	58.6% <58.6%> (ø)
internal/adminapi/kong.go	60.0% <60.0%> (ø)
internal/manager/setup.go	52.6% <61.5%> (-1.9%)	⬇️
internal/admission/server.go	42.0% <76.9%> (+1.1%)	⬆️
internal/manager/config_validation.go	90.6% <88.2%> (-9.4%)	⬇️
... and 8 more

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[randmonkey]

Generally good, only some small comments reamains.

[pmalek]

👍 ( I already feel the pain resolving conflicts: #3421)

Just make sure to resolve all the conversations also from @randmonkey

[czeslavo]

I already feel the pain resolving conflicts: #3421

Oh oh, sorry for the inconvenience. 🙁

Just make sure to resolve all the conversations also from @randmonkey

✅