Add support for email notifications configurations and possibly email notification functionality

[colinjlacy]

In order to accommodate gianlucam76/k8s-cleaner#51 we'll need to adda new notification type to the possible configuration options that libsveltos picks up on.

Currently, k8s-cleaner uses types from the libsveltos alpha API. Adding a new reserved key(s) to allow specification of email configuration data in K8s secrets will require adding keys to the API files here.

Conversions with @gianlucam76 also involved adding email notification support to libsveltos as well, so that the functionality could be centrally located and usable by all projects that leverage this library. Doing so would require:

• ensuring local secrets can be read by libsveltos (may mean porting the code from k8s-cleaner into this repo)
• adding a new NotificationType in this repo
• setting and documenting reserved secret data key-value pairs for email configuration
• building out email notification functionality, likely using the built-in net/smtp lib