fix(controller): weighted experiment validation should allow delegating to trafficRouter plugins

[bezmax]

Even though weighted experiment calls are correctly delegated to Traffic Router plugins, the validation is currently not allowing to register a TrafficRouting plugin when weighted experiments are present.

It seems that this validation was added before plugin architecture supported additionalDestinations, so it made sense then. Ideally a plugin would have a way to announce it's capabilities, but given right now API does not have anything suitable for that - this PR provides a simple way to unblock using plugins that support weighted experiments.

Checklist:

• Either (a) I've created an enhancement proposal and discussed it with the community, (b) this is a bug fix, or (c) this is a chore.
• The title of the PR is (a) conventional with a list of types and scopes found here, (b) states what changed, and (c) suffixes the related issues number. E.g. "fix(controller): Updates such and such. Fixes #1234".
• I've signed my commits with DCO
• I have written unit and/or e2e tests for my change. PRs without these are unlikely to be merged.
• My builds are green. Try syncing with master if they are not.
• My organization is added to USERS.md.

[sonarcloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarCloud

[github-actions]

Published E2E Test Results

  4 files    4 suites   3h 15m 28s ⏱️
113 tests 103 ✅  7 💤 3 ❌
456 runs  424 ✅ 28 💤 4 ❌

For more details on these failures, see this check.

Results for commit 0450cd4.

[github-actions]

Published Unit Test Results

2 279 tests   2 279 ✅  2m 59s ⏱️
  128 suites      0 💤
    1 files        0 ❌

Results for commit 0450cd4.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 82.70%. Comparing base (1264f8d) to head (0450cd4).
Report is 5 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3909      +/-   ##
==========================================
+ Coverage   82.69%   82.70%   +0.01%     
==========================================
  Files         163      163              
  Lines       22886    22886              
==========================================
+ Hits        18925    18928       +3     
+ Misses       3087     3085       -2     
+ Partials      874      873       -1     

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