[mnet32] Publish Internal State

[Mischback]

MOVED FROM #5

This was originally part of #5, but to actually determine which information is required by other modules of the whole project, these other components have to be - at least - stubbed out. To make clear that the basic functions of #5 are already in place, these things are moved into this issue.

• publish internal state of embedded_networking_esp32 (mnet32)
  • if connected to a network:
    • signal strength
    • SSID
    • assigned (or statically configured) IP
    • actual internet access?
  • if in access point mode:
    • SSID of the network
    • password / PSK Design Decision: No disclosure of passwords!
      • should be configurable, if this information is to be disclosed (at build time)
    • remaining time of the availability of the access point (see above)

[Mischback]

Component-specific Status Information

• Status Information is not permanently available, but will be created as required.
• Status Information is published using ESP-IDF's event library
• Status Information is published periodically from networking() with a configurable (sdkconfig, pre-build) frequency
• Status Information might be requested from other components
  • triggers publishing outside of periodic updates, uses same function

struct networking_public_status {
    networking_medium medium;    // directly from internal state
    networking_mode   mode;      // directly from internal state
    networking_status status;    // directly from internal state
    char              ssid[32];  // There is a constant for the maximum length!
                                 // Either the SSID of the WiFi network connected to or
                                 // the SSID of the access point
                                 // For convenience, this might be set to "WIRED" if
                                 // ``medium`` is ``ETHERNET``
    // ip mode                   // ``dhcp`` or `static``
    // ip address (IPv4)         // Either the assigned IP-address or the (automatic) IP
                                 // address in access point mode
    // ip address (IPv6)
    // netmask (IPv4 only)
    // default gw (IPv4)         // Is this relevant for IPv6?!
    // nameserver (IPv4)         // Is this relevant for IPv6? Yes, most likely another 
                                 // field is required
};