Restructure HTTP1-specific code and add invocation strategy

Restructures HTTP1-specific operation types into a separate library to allow SmokeOperations to be protocol-agnostic. Also adds an invocation strategy that by default hands off each request to DispatchQueue.global(). This avoids running operations on the thread from SwiftNIO and running into issues with the event loop. Provides an extension point with a strategy protocol if required.

Note: This is a breaking change. The package import for HTTP1-specific operation types will need to change from SmokeOperations to SmokeOperationsHTTP1.

Initial release of Smoke Framework

Initial release of Smoke Framework. See the fronting README for details on features.