Add capability-gated ESP32 download reboot

[TurkeyMan]

What changed

• advertises download-mode reboot support as a driver capability
• implements classic ESP32 download reboot by holding RTC GPIO0 low before esp_restart()
• extends /system/reboot with an optional bootloader argument only on capable targets
• explicitly reports the capability as unavailable on other current system backends

Why

Boards without convenient access to their boot strap controls need a software path into the ROM downloader for recovery and development. Capability gating keeps the command portable rather than assuming every platform supports it.

Validation

• cherry-picked cleanly onto current master
• compiled in the successful SmartEVSE ESP32 release image
• default Linux LDC release build passes with unsupported capability handling
• verified the classic ESP32 RTC GPIO API and enum values against the installed ESP-IDF 6 headers
• git diff --check passes

Hardware invocation remains to be smoke-tested on a classic ESP32 board.