feat(examples): Add esp_netif examples demonstrating network interface configurations

[abhik-roy85]

Description

This commit adds new examples showcasing different esp_netif network interface configurations and use cases.

Added examples:

• eth_gateway: Ethernet Access Point example that runs a DHCP server on a single Ethernet interface, allowing connected devices to obtain IP addresses automatically
• eth_gateway_wifi_ap: Dual AP configuration example that simultaneously operates both Ethernet and Wi-Fi interfaces as Access Points with DHCP servers, enabling the ESP to act as a multi-interface router providing both wired (192.168.5.x) and wireless (192.168.4.x) network access simultaneously. Includes configurable DHCP pools, DNS support, NAPT routing, and client management for both interfaces."
• eth_gateway_wifi_sta: Hybrid configuration example that operates Ethernet as an Access Point with DHCP server while simultaneously running Wi-Fi as a Station connecting to an external access point, enabling network bridging between wired and wireless networks
• eth_endpoint_wifi_sta: Dual interface example that simultaneously operates both Ethernet STA and Wi-Fi STA interfaces as DHCP clients
• eth_endpoint_wifi_ap: Hybrid configuration example that operates Ethernet as a Station with DHCP client while simultaneously running Wi-Fi as an Access Point with DHCP server, enabling network bridging between wired upstream connection and local wireless network

Related

• #14052

Testing

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Checklist

Before submitting a Pull Request, please ensure the following:

• 🚨 This PR does not introduce breaking changes.
• All CI checks (GH Actions) pass.
• Documentation is updated as needed.
• Tests are updated or added as necessary.
• Code is well-commented, especially in complex areas.
• Git history is clean — commits are squashed to the minimum necessary.

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Note

Low Risk
New code is isolated to examples and a .gitignore tweak, with no changes to production components or security-sensitive logic.

Overview
Adds two new esp_netif example projects under examples/esp_netif: EthAp configures a single Ethernet interface with a static IP and runs an on-device DHCP server (including configurable lease time, address pool, and optional DNS offers), and EthStaWifiSta brings up Ethernet STA and Wi‑Fi STA concurrently as DHCP clients while logging IP/DNS info and waiting for Wi‑Fi to obtain an address.

Also updates .gitignore to ignore clangd cache/index artifacts (.cache/, *.idx).

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[kostaond]

Is the only difference to EthAPWifiAP the fact that Ethernet is set as DHCP Client? If so, I would merge the examples where it makes sense together and rename them to something more common and make DHCP Server/Client as configurable option in the menuconfig. Otherwise, it will be nightmare to maintain.

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[david-cermak]

LGTM!

I think it's in a merge-able state, but I'd only add build only jobs for the apps, so we keep it buildable.

[alisitsyn]

LGTM!