Add serial ports config file

[SergioGasquez]

Don't really like the way I solved this as it's basically a bunch of duplicated code, but not sure if there's any better way.

Draft until I do some more testing and/or I find a better solution.

Example espflash_ports.toml file:

[connection]
serial = "/dev/ttyUSB0"

[jessebraham]

Not sure this is the path we want to take to be honest. Had some other ideas I've been meaning to explore, so let's sit on this for awhile please.

[JurajSadel]

I wonder if we can test this somehow in HIL

[MabezDev]

It seems to me that we can avoid the code duplication by loading the two things separately then combining them again in code. We just need to be careful when saving the config again to write out the config back to the two separate files. After that, I think this becomes a relatively simple change?

Here is a small snippet to explain what I mean:

// package both together for code locality reasons, but they are seperate configs which are saved seperately
struct Config {
  config: ProjectConfig, // currently this is Config, let's rename it
  port_config: PortConfiguration // user port config
} 

What do you think?

[SergioGasquez]

Only PortConfig requires save_path, as its the only config file that can be saved, save_with is only called here: https://github.com/esp-rs/espflash/blob/main/espflash/src/cli/serial.rs#L53

[SergioGasquez]

What do you think?

Definitely better, thanks for the suggestion @MabezDev, just updated it!