Prioritise API with api write=on

[Akshay2191]

Issue:
Currently, if more than one Plus API is defined in NGINX config, Agent will always select the first working API URL, and if that API has  "api write=off", that leads to "MethodDisabled" error while trying to update upstream using an API.

Fix:
With this change agent is parsing all the Plus API's defined in the NGINX config to find if any of them have "api write=on" and if yes, then we are selecting that as the default api; else we are selecting the first API defined in the NGINX config as the default one, which is the current behaviour.

Checklist

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• If applicable, I have added tests that prove my fix is effective or that my feature works
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• 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