MCUBoot Agent Feature. Add extra headers.

When downloading images in mcuboot_agent application from a URL with a GET command it would be a good feature to add a function that lets you send board custom headers to the server to indicate board related data.

I am thinking in a OTA Server that serves the firmware image depending in the board ID, Firmware current version, etc...

A function that adds to the` webclient_context struct` the extra headers could be implement in `mcuboot_agent_main.c` file in `download_firmware_image` function like this:

```
#ifdef CONFIG_MCUBOOT_UPDATE_AGENT_EXTRA_HEADERS
mcuboot_add_extra_headers(&client_ctx)
#endif
```
Then `mcuboot_add_extra_headers` would be defined by the user depending on his concrete server according to its needs...

How do you see this feature?? Could it be a good idea?? @pkarashchenko