Add Smtp project

[clackary]

Pull Request Description

Adds Smtp project to the suite - https://github.com/Mikroservices/Smtp.

Note that a recent compiler regression was discovered when building this project in release mode. swiftlang/swift#64219

Acceptance Criteria

To be accepted into the Swift source compatibility test suite, a project must:

• be an Xcode or swift package manager project
• support building on either Linux or macOS
• target Linux, macOS, or iOS/tvOS/watchOS device
• be contained in a publicly accessible git repository
• maintain a project branch that builds against Swift 4.0 and passes any unit tests
• have maintainers who will commit to resolve issues in a timely manner
• be compatible with the latest GM/Beta versions of Xcode and swiftpm
• add value not already included in the suite
• be licensed with one of the following permissive licenses:
  • MIT
• pass ./project_precommit_check script run

Ensure project meets all listed requirements before submitting a pull request.

PASS: Smtp, 5.0, e8f09e, Swift Package
========================================
Action Summary:
     Passed: 1
     Failed: 0
    XFailed: 0
    UPassed: 0
      Total: 1
========================================
Repository Summary:
      Total: 1
========================================
Result: PASS
========================================

[clackary]

Might be a step ahead of myself, but I've added the xfail because the project doesn't currently build in recent toolchains.

[clackary]

CC @mczachurski - in case you wanted to keep an eye on this.

[clackary]

Removed the xfail since a fix has landed on main.

[justice-adams-apple]

@swift-ci test