Consider an `HTTPVersion` type

Vapor and NIO both have an `HTTPVersion` type and I imagine the new HTTP server will want one as well. Most clients and server will need a currency type to determine/represent the current version they're talking. Is this something that would be welcome in the repo? It seems silly to have multiple representations around if we deem this a core type. Happy to do a PR if there's interest for this

The issue names no repository files or tests. Start by comparing the HTTPVersion types in Vapor and NIO, then clarify the requirements for a shared type used by HTTP clients and servers; done means the repository has an agreed representation and scope for the proposed core type.

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