Support for WPA2 Enterprise

[jesmith-all]

• Adds optional argument to WifiConnectionHandler constructor to pass a username
• Conditional in .cpp checks if value has been set, calls on .begin or .beginEnterprise accordingly
• Critical to use at universities
• If merged, would allow students on university networks to use IoT Cloud without modifying a local copy of the library

[CLAassistant]

All committers have signed the CLA.

[facchinm]

Hi @jesmith-all ,
thanks for the PR! However, the API is a bit confusing IMO; I'd prefer a derived class similar to #63 that could be "selected" by the user as soon as arduino-libraries/ArduinoIoTCloud#265 get merged and the online "stub" generator updated.
This would allow calling beginEnterprise() with all the different options (like entity/ca if needed).

[jesmith-all]

@facchinm (@brownby)

Fair enough. With an active course now and more around the corner, I sought the path of least resistance in edits that would add the functionality as a stop gap, but you're right that this isn't as clean as it could be.

I have not been tracking these other developments, but it's great to hear that you've got a vision for how things could coalesce. I'll take a look and circle back.

[olipinski]

@facchinm Does it still need to be in a derived class? I see that the project file structure has changed quite a bit since this PR, but if it still needs to be a separate class, I can give it a go.