Fix WiFi credentials clear event not being fired

[garzdin]

Problem

A WiFi credentials event should be fired when a user calls WiFi.clear_credentials(). It doesn't currently happen.

Solution

A system event is triggered, when a user clears WiFi credentials.

Example App

void credentialsChanged() {
    Serial.println("Credentials changed");
}

SYSTEM_MODE(AUTOMATIC);

void setup() {
    Serial.begin(9600);
    System.on(network_credentials, credentialsChanged);
}

void loop() {
    Serial.println("Clearing credentials");
    WiFi.clearCredentials();
    delay(1000);
}

References

#1570

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Completeness

• User is totes amazing for contributing!
• Contributor has signed CLA (Info here)
• Problem and Solution clearly stated
• Run unit/integration/application tests on device
• Added documentation
• Added to CHANGELOG.md after merging (add links to docs and issues)

[m-mcgowan]

I feel I would like to make this conditional on the result of wlan_clear_credentials. We should also check if other Radio interfaces need a similar change. It might be better to add it to the network_clear_credentials() function so that it is ubiquitous?