Extract e-mail transport from sink

[mgrosperrin]

I have introduced two abstractions:

• Email: a simple DTO that represent the need of the sink for an e-mail (From, Tos, Subject, Body, IsBodyHtml)
• IEmailTransport: responsible of converting the Email abstraction to the concrete mail type, create the SmtpClient and send the e-mail throught it.

I modified the EmailConnectionInfo to add a internal virtual CreateEmailTransport method to create the IEmailTransport the sink should use. This will allow unit testing the sink without the transport.

There is now only one sink and two implementations of the transport.

Regarding PRs #51 and #54, I think it will be simpler to merge this one first and update the others PRs .

fixes #27

PS: I also fix a typo when splitting the To addresses... PPS: this replaces the PR #55

[mgrosperrin]

I introduce, on the tests project, a new xUnit.Net attribute (UseCultureAttribute) to set the culture for the tests and a new constant (NEW_ARGUMENTOUTOFRANGEEXCEPTION_MESSAGE) to handle change of message for exception message in .NET Core 3.1

[nblumhardt]

Awesome, thanks @mgrosperrin!

Made a note regarding the origin of UseCultureAttribute, since it's third-party code.

On its way through CI to a NuGet dev package now 😎

[mgrosperrin]

Awesome, thanks @mgrosperrin!

Made a note regarding the origin of UseCultureAttribute, since it's third-party code.

On its way through CI to a NuGet dev package now 😎

Hi, Thanks for the note, the attribute is my codebox but I forgot to add the origin 😔.