Add checks to see if stream endpoints exist before calling them

[dhurley]

Proposed changes

This changes is required for the use case where someone doesn't have streams configured in their nginx.conf and want to prevent 404 errors from being logged in the NGINX access log.

127.0.0.1 - - [05/Oct/2023:13:10:29 +0000] "GET /api//8/stream/upstreams HTTP/1.1" 404 183 "-" "Go-http-client/1.1" "-"
127.0.0.1 - - [05/Oct/2023:13:10:29 +0000] "GET /api//8/stream/zone_sync HTTP/1.1" 404 183 "-" "Go-http-client/1.1" "-"
127.0.0.1 - - [05/Oct/2023:13:10:29 +0000] "GET /api//8/stream/limit_conns HTTP/1.1" 404 183 "-" "Go-http-client/1.1" "-"
127.0.0.1 - - [05/Oct/2023:13:10:44 +0000] "GET /api//8/stream/server_zones HTTP/1.1" 404 183 "-" "Go-http-client/1.1" "-"

Checklist

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

• I have read the CONTRIBUTING doc
• I have added tests that prove my fix is effective or that my feature works
• I have checked that all unit tests pass after adding my changes
• I have updated necessary documentation
• I have rebased my branch onto main
• I will ensure my PR is targeting the main branch and pulling from my branch from my own fork

[oliveromahony]

Please add sufficient tests and the log entries which led to these changes