fix(serde_at): deserialize _

[denysvitali]

While parsing the CGMI of my device (SIM7020), I've noticed that I was getting a lot of parsing errors.
After digging deeper, I've discovered that the parsing errors were actually EofWhileParsingString errors.

Finally, I've discovered why: my manufacturer string looked like this:

SIMCOM_Ltd

And this was breaking at the _, because it's not considered alphanumeric.
My fix here is to allow all ASCII characters >= 32 (space + printable characters).

The only non-printable character included is DEL (127) - but it shouldn't be a big deal.

---

match client.send(&common::GetManufacturerId{}).await {
    Ok(resp) => {
        info!("Manufacturer ID OK: resp={:?}", resp.id.as_str());
    }
    Err(e) => {
        println!("Error: {:?}", e);
    }
}

Before my fix:

DEBUG Response: "SIMCOM_Ltd"
Error from serde_at: EofWhileParsingString

After my fix:

DEBUG Response: "SIMCOM_Ltd"
INFO Manufacturer ID OK: resp="SIMCOM_Ltd"