EmailSenderProvider with global defaults for SMTP settings

[xgp]

Implementation of EmailSenderProvider which behaves as the default, unless you specify variables to configure provider defaults. In this case, any realm that does not have an SMTP server set up will default to use the values set in the variables. This is useful in environments where a single SMTP server is used by many realms, and the Keycloak administrator does not want to distribute credentials to every realm administrator.

This can also be useful in environments where you want to allow realms to "test" Keycloak's email sending without having to configure an SMTP server. For this use case, include a counter in the distributed cache that is used to limit the number of emails that are sent using the global configuration, in order to prevent spammers from exploiting the free email capability. This can be configured with the max-emails variable.

Started in #41