Enhance GD32 flasher with device probing

Adds GD32 JSON database support to resolve series and part numbers from chip ID and flash size, including register-bit probing for ambiguous families. Extends bootloader functionality with memory reads, flash size/UID retrieval, GO command support, new CLI flags (`--get-size`, `--get-uid`, `--db`, `--go`, `--go-address`), and tighter error handling for serial reads.

Author: vanvught