Add OAuth2 and fingerprint verification to email examples

[Tiogaplanet]

Is your feature request related to a problem? Please describe.
No, it's related to security. The Gmail notifications require use of less secure apps from Google and the SHA-1 fingerprint verification is not being done in the sketch.

Describe the solution you'd like
The email notification examples should support OAuth2 and SHA-1 or SHA-2 fingerprint verification.

Describe alternatives you've considered
Using other email providers that explicitly allow less secure access with user/password pairs.

Additional context
None.

[Tiogaplanet]

An esp8266 and oauth2 proof of concept

Used in this tutorial:
Using Gmail with OAUTH2 in Linux and on an ESP8266