Reduce the number of times env.Processes gets called

[dhurley]

Proposed changes

• Only call env.Processes once on startup of the agent
• Plugins that require process information now subscribe to core.NginxDetailProcUpdate to get any updates
• The process watcher plugin now only checks for process changes every 5 seconds instead of every 500 milliseconds

Checklist

Before creating a PR, run through this checklist and mark each as complete.

• I have read the CONTRIBUTING document
• I have run make install-tools and have attached any dependency changes to this pull request
• If applicable, I have added tests that prove my fix is effective or that my feature works
• If applicable, I have checked that any relevant tests pass after adding my changes
• If applicable, I have updated any relevant documentation (README.md)
• If applicable, I have tested my cross-platform changes on Ubuntu 22, Redhat 8, SUSE 15 and FreeBSD 13

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[codecov-commenter]

Codecov Report

Attention: 18 lines in your changes are missing coverage. Please review.

Comparison is base (d75b51e) 66.40% compared to head (ff37049) 66.25%.
Report is 1 commits behind head on main.

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Additional details and impacted files

@@            Coverage Diff             @@
##             main     #482      +/-   ##
==========================================
- Coverage   66.40%   66.25%   -0.16%     
==========================================
  Files         118      118              
  Lines       13392    13412      +20     
==========================================
- Hits         8893     8886       -7     
- Misses       3907     3935      +28     
+ Partials      592      591       -1     

Files	Coverage Δ
...r/github.com/nginx/agent/v2/src/plugins/metrics.go	68.06% <100.00%> (+1.13%)	⬆️
...dor/github.com/nginx/agent/v2/src/plugins/nginx.go	71.42% <100.00%> (-0.21%)	⬇️
....com/nginx/agent/v2/src/plugins/process_watcher.go	87.09% <100.00%> (-0.14%)	⬇️
...hub.com/nginx/agent/v2/src/plugins/registration.go	79.02% <100.00%> (+0.14%)	⬆️
...or/github.com/nginx/agent/v2/src/plugins/common.go	70.93% <88.88%> (+0.69%)	⬆️
...com/nginx/agent/v2/src/plugins/dataplane_status.go	67.28% <71.42%> (-0.48%)	⬇️
.../github.com/nginx/agent/v2/src/plugins/features.go	36.79% <45.45%> (-0.21%)	⬇️
...github.com/nginx/agent/v2/src/plugins/agent_api.go	61.93% <50.00%> (-0.87%)	⬇️

... and 3 files with indirect coverage changes

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[oliveromahony]

can you regenerate the profile.cgo?

1. Run agent on a production system with pprof enabled (for about an hour, run through some of the workflows, metrics, config push etc).
2. Take a cpu profile from the REST API (call it cpu.prof)
3. Run the performance tests with cpu profiling:

test-bench: ## Run benchmark tests
	cd test/performance && GOWORK=off CGO_ENABLED=0 go test -mod=vendor -cpuprofile metrics_test_cpu.prof -count 5 -timeout 2m -bench=. metrics_test.go
	cd test/performance && GOWORK=off CGO_ENABLED=0 go test -mod=vendor -cpuprofile user_workflow_test_cpu.prof -count 1 -bench=. user_workflow_test.go
	cd test/performance && GOWORK=off CGO_ENABLED=0 go test -mod=vendor -cpuprofile plugins_test_cpu.prof -count 5 -timeout 2m -bench=. plugins_test.go
	cd test/performance && GOWORK=off CGO_ENABLED=0 go test -mod=vendor -cpuprofile environment_test_cpu.prof -count 5 -timeout 2m -bench=. environment_test.go

4. Rename profile.cgo to profile.pprof
5. Merge profile.pprof with cpu.prof, metrics_test_cpu.prof user_workflow_test_cpu.prof environment_test_cpu.prof metrics_test_cpu.prof
   go tool pprof -proto profile.pprof metrics_test_cpu.prof user_workflow_test_cpu.prof environment_test_cpu.prof metrics_test_cpu.prof > profile.cgo