In `FlashArgs`, `monitor_args` requires `monitor` arg, but clap does not support `requires` with `flatten`

[SergioGasquez]

See https://github.com/esp-rs/espflash/pull/737/files#r1937149668

[SergioGasquez]

We should also investigate if there is a better way to check that elf is provided if using log-format in the monitoring subcommand and to now allow something like flash --monitor target/.../elf --elf target/.../elf. See #759 (comment)