Triggers for wifi/api connect/disconnect

**Describe the problem you have/What new integration you would like**

In many cases we need to execute code when Wi-Fi connection (or HA api) is lost / restored.

Currently there are no triggers for these events:

- `wifi.on_connect`
- `wifi.on_disconnect`
- `api.on_connect`
- `api.on_disconnect`

A recent PR (#746) added something similar for MQTT only.

**Please describe your use case for this integration and alternatives you've tried:**

Many examples found on internet suggest a workaround like:

```yaml
interval:
- interval: 1s
then:
- if:
condition:
- not: wifi.connected
then:
- (...)
- if:
condition:
- wifi.connected
then:
- (...)
```

While the above code works, polling is not the best way to solve this problem. A trigger could (presumably) reduce cpu usage.

**Additional context**

The above workaround has also two additional problems:

- The user is left to implement its own way to detect a _change_ in the state (otherwise the code will repeatedly run each 1s)
- The code is a bit verbose

All this could be solved with appropriate triggers for each event.