fix(net): Rework NO_GLOBALS to not act on dependent network classes

[me-no-dev]

This pull request standardizes the way global instances are conditionally defined across several network-related libraries. Specifically, it ensures that global objects like WiFi, ETH, PPP, Network, and MDNS are only instantiated when the appropriate macros are not defined, improving consistency and configurability throughout the codebase.

Conditional global instance definitions:

• Added #if !defined(NO_GLOBAL_INSTANCES) && !defined(NO_GLOBAL_ETH) guard around the global ETH instance in ETH.cpp for consistency with other modules.
• Added #if !defined(NO_GLOBAL_INSTANCES) && !defined(NO_GLOBAL_PPP) guard around the global PPP instance in PPP.cpp.
• Removed unconditional global instance declarations and ensured that WiFi, Network, and MDNS are only declared when the appropriate macros are not defined in their respective header files (WiFi.h, NetworkManager.h, ESPmDNS.h). [1] [2] [3]

Closes: #11871

[github-actions]

Test Results

 76 files   76 suites   15m 7s ⏱️
 38 tests  38 ✅ 0 💤 0 ❌
241 runs  241 ✅ 0 💤 0 ❌

Results for commit 9e58504.

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[github-actions]

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