Use state file for updating N+ upstreams

[sjberman]

Problem: When splitting the data and control plane, we want to be able to maintain the ability to dynamically update upstreams without a reload if using NGINX Plus. With the current process, we write the upstreams into the nginx conf but don't reload, then call the API to actually do the update. Writing into the conf will trigger a reload from the agent once we split, however.

Solution: Don't write the upstream servers into the nginx conf anymore when using NGINX Plus. Instead, utilize the state file option that the NGINX Plus API will populate with the upstream servers. This way we can just call the API and don't unintentionally reload by writing servers into the conf.

Testing: Verified that the state file is set and populated when using NGINX Plus. With OSS, servers are still written to the config.

Closes #2841

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• I have added tests that prove my fix is effective or that my feature works
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[codecov]

Codecov Report

Attention: Patch coverage is 90.24390% with 4 lines in your changes missing coverage. Please review.

Project coverage is 89.67%. Comparing base (245b516) to head (751674d).

Files with missing lines	Patch %	Lines
internal/mode/static/handler.go	87.50%	2 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2888      +/-   ##
==========================================
- Coverage   89.71%   89.67%   -0.05%     
==========================================
  Files         109      109              
  Lines       11091    11077      -14     
  Branches       50       50              
==========================================
- Hits         9950     9933      -17     
- Misses       1082     1084       +2     
- Partials       59       60       +1     

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