Add Captive Portal Example

[fhessel]

This PR introduces a very basic captive portal example. It only makes use of DNS redirects and answers only on HTTP.

There would be two great extensions to this, but they are hard to implement in Arduino or for the given scenario and therefore not implemented:

• RFC7710bis introduces a DHCP option for captive portal URLs, but without the full lwip/network stack from the ESP-IDF, it seems impossible to modify the behavior of the DHCP server, and that would not make a nice example anyway.
• The Captive Portal API requires HTTPS and valid certificates (both MUST-statements in the specification), which will not match the regular use case (setting up WiFi of the ESP32 etc.).

References

• Closes #97