add http/2 server support (h2)

[tanner0101]

This PR adds support for h2 aka HTTP/2 over TLS via ALPN. To enable HTTP/2 support, a new field supportVersions has been added to HTTPServerConfig.

let config = HTTPServerConfig(
    hostname: hostname,
    port: 8443,
    supportVersions: [.one, .two],
    tlsConfig: .forServer(...)
)

Here are the type declarations:

struct HTTPServerConfig {
    ...
    var supportVersions: Set<HTTPVersionMajor>
    ...
}

public enum HTTPVersionMajor: Equatable, Hashable {
    case one
    case two
}

When .two appears in the supportVersions set, the HTTP server will add h2 to its supported ALPN protocols list. If the HTTP/2 client decides to use h2, the server will configure HTTP/2 streams similar to HTTP/1.1 TCP connections: each stream will get its own delegate handler.

By default, HTTPServerConfig will specify support for both HTTP/1.1 and HTTP/2.

Todo:

• [ ] h2c aka HTTP/2 over TCP support. (Probably a separate PR)
• [ ] HTTP upgrade support over h2 aka websocket
• [ ] HTTPClient support
• [x] Find solution for https://github.com/apple/swift-nio-ssl/pull/70

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