enh(apps::monitoring::zscaler::zdx::api): adapt to OneAPI authentication

[omercier]

Description

Authentication to Zscaler APIs now all go through OneAPI (read heare for more information).

Since this is now the only way to access this API, the plugin had to be adapted.

Refs: CTOR-2251

Type of change

• Patch fixing an issue (non-breaking change)
• New functionality (non-breaking change)
• Functionality enhancement or optimization (non-breaking change)
• Breaking change (patch or feature) that might cause side effects breaking part of the Software

Summary by Aikido

Security Issues: 0	Quality Issues: 0	Resolved Issues: 0

⚡ Enhancements

• Replaced legacy auth with OneAPI client_credentials flow and options
• Updated packaging and Dockerfiles to add dependencies and enable repos

🔧 Refactors

• Changed token caching and retrieval to SHA256-based cache key
• Hardened API responses with safe defaults and missing-result handling

^{More info}