Enable Builder Pattern for TypeSafeMiddleware

[herzi]

This change will allow using the builder pattern with TypeSafeMiddleware.

Description

The builder pattern is a useful way of writing compact code in a functional way. Regular middleware can already utilize this from RouterHTTPVerbs_generated.swift like this:

let router = Router()
  .get("products") { _, _, next in next() }
  .post("products") { _, _, next in next() }

Unfortunately, it's not possible to mix and match with type safe middleware.

Motivation and Context

This change increases consistency within Kitura by allowing to utilize method chaining with type safe middleware just as Kitura permits for regular middlewares.

How Has This Been Tested?

• This code has been modified by me using Xcode and the changes are pretty obvious.
• I have – however – updated my master branch to contain both a simple Github Action and the changes from the feature branch. Feel free to check out the results.

Checklist:

• If applicable, I have updated the documentation accordingly.
• If applicable, I have added tests to cover my changes.
  • Not sure about this one. I haven't added tests. If you like me to add one or two, just tell me.

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[herzi]

It looks to me like the Travis CI setup is broken for some configurations:

• There is a 404 error happening when downloading a Swift toolchain: https://travis-ci.org/github/IBM-Swift/Kitura/jobs/698255369