* Implement IEmailSenderClient interface and implementation
In an effort to support oauth 2 and other schemes, we extract a emailsenderclient interface, allowing to replace default smtp client with one that fits the usecase, without having to implement all of Umbracos logic that builds the mimemessage
* fix test
* Documentation
* EmailMessageExtensions public, use EmailMessage in interface and impl.
* move mimemessage into implementation
* revert EmailMessageExtensions back to internal
* use StaticServiceProvider to avoid breaking change
* Fix test after changing constructor
* revert constructor change and add new constructor an obsoletes
* Moved a paranthesis so it will build in release-mode
(cherry picked from commit bff321e6f5)
7cc8f844f7