system/curl: add a small curl-like HTTP client command

[JorgeGzm]

Summary

Add a small curl-like HTTP client command under system/curl, built on top
of the netutils/webclient library. It brings the familiar curl option
syntax to the NuttShell so HTTP servers can be exercised directly from the
target.

Supported options (a subset of the real curl, see https://curl.se/docs/manpage.html):

• -X <method> : HTTP method (defaults to GET, or POST when a body is sent)
• -H <header> : add a request header
• -d <data> / -d @file : raw request body (inline or read from a file)
• -F name=@file : multipart/form-data file upload
• -o <file> : write the response body to a file
• -v : verbose

HTTP only (no HTTPS).

Impact

• New optional command guarded by CONFIG_SYSTEM_CURL (default n); no impact
  when disabled.
• Depends on CONFIG_NETUTILS_WEBCLIENT / CONFIG_NET_TCP.
• No change to any existing application.

Testing

• Build Host: Linux, arm-none-eabi-gcc
• Target: linum-stm32h753bi:curl (companion NuttX PR adds this config), STM32H753BI over Ethernet + SD card
• nxstyle clean on system/curl/curl_main.c.

Runtime on hardware:

nsh> ifconfig
eth0    Link encap:Ethernet HWaddr 00:e0:de:ad:be:ef at RUNNING mtu 1486
        inet addr:192.168.15.3 DRaddr:192.168.15.1 Mask:255.255.255.0

GET, saving the response to a file (-o):
nsh> curl -o /mnt/dl.json http://httpbin.org/json
curl: HTTP 200
nsh> curl -o /mnt/img.png http://httpbin.org/image/png
curl: HTTP 200
nsh> ls -l /mnt/img.png
 -rw-rw-rw-        8090 /mnt/img.png

POST JSON (-d selects POST, -H sets the type); httpbin echoes it back:
nsh> curl -H Content-Type:application/json -d '{"temp":25.3,"hum":60}' http://httpbin.org/post
{
  "data": "{\"temp\":25.3,\"hum\":60}",
  "json": { "hum": 60, "temp": 25.3 },
  "url": "http://httpbin.org/post"
}
curl: HTTP 200

Custom method (-X):
nsh> curl -X PUT -d '{"x":1}' http://httpbin.org/put
curl: HTTP 200

Multipart upload (-F name=@file) and download back (-o):
nsh> curl -F file=@/mnt/nuttx_logo.png "http://192.168.15.12:8000/api/.../ota/upload?version=logo1"
{"filename":"nuttx_logo.png","sha256":"ca278dcb...","size_bytes":40343}curl: HTTP 201
nsh> curl -o /mnt/logo_back.png http://192.168.15.12:8000/api/.../ota/default/logo1/nuttx_logo.png
curl: HTTP 200
nsh> ls -l /mnt
 -rw-rw-rw-       40343 nuttx_logo.png
 -rw-rw-rw-       40343 logo_back.png

[cederom]

Thank you @JorgeGzm :-)

[acassis]

@xiaoxiang781216 maybe now we can remove the netutils/libcurl4nx that Sebastien started to implement and never finished