Add package for MailKit

[bdukes]

Fixes #4563

I haven't had a chance to test that this does the right thing, waiting for the generated packages to check.

Also, I'd like to adjust how we're generating these to read the versions and update the manifest magically, but haven't gotten to that yet.

[mitchelsellers]

@bdukes I think the only other thing we need to do here is also exclude these files from the created install/upgrade packages correct?

[bdukes]

@mitchelsellers they're excluded from the packages in Build/Tasks/packaging.json

[bdukes]

Alright, I think this is ready for review. The manifest version and assembly versions are automatically set during packaging. I've done an install and an upgrade from 9.8.1 to 9.9.1 and verified that the MailKitMailProvider works on both sites.

One thing to note is that I did not get a MailKitMailProvider entry added to the web.config upon upgrade, I had to add it manually. I don't think that's related to these changes, but perhaps another issue we need to investigate.

[valadas]

[x] v9.9.1 Clean install: Emails work and mailkit is available but not set a default
[x] v9.9.0 pre-configures for MailKit, stays configured with MailKit and still works
[ ] v9.1.1 upgrade emails keep working but MailKit is not available in the web.config providers. What is the plan for this ? merge this PR and do another one for that ?

Other than that, it looks good to me.

[valadas]

Looks good to me if we handle the upgrade adding of the provider in the web.config separatelly...

[valadas]

I think #4582 will fix the web.config part of this, waiting for the build to confirm that...