Reboot a single instance within a rental

Reboots one instance. Call it only for an instance whose instances[].supportsReboot is true. Virtual machines get a warm in-place guest reset; bare-metal servers are power-cycled. Disk contents are preserved either way; unsaved in-memory work is lost. Reports INVALID_INSTANCE_REF for an instanceRef outside this rental, REBOOT_NOT_SUPPORTED for a rental that does not support reboot, and REBOOT_ALREADY_IN_PROGRESS while one is in flight.