Add http/2 support to HTTPBin

[fabianfett]

This PR adds http/2 support to the HTTPBin test utility.

Note regarding semver:

This PR adds swift-nio-http2 as a dependency.

Modifications

• Removed obscure initialization options for HTTPBin
  • channelPromise: EventLoopPromise<Channel>? = nil was only used once (in testUploadStreamingBackpressure).
  • connectionDelay: TimeAmount = .seconds(0) was never used
  • maxChannelAge: TimeAmount? = nil was never used
• The initialization options ssl: Bool = false, compress: Bool = false, refusesConnections: Bool = false have moved into a Mode enum. The mode might be .http1_1(ssl: Bool, compress: Bool), .http2(compress: Bool), or .refuse
• Added support for http/2 (not used in any test yet, however I verified the support with curl locally.)
• channel pipeline modifications are nearly all synchronous.
• the HTTPBin's request handler can be changed. This allows testing of more esoteric cases without having to create a new server every time. The default HTTPBin handler continues to use the HTTPBinHandler.

Result

• Easier test utils that can be used for http/2

Follow ups

• I also wanted to add a ServerQuiescingHelper to ensure that all connections are closed when the test ends. However we often seem to leak connections when we cancel tasks. This would make our CI very flaky. I hope to land this at a later point once all connection and connection pool improvements have landed (See [RFC] ConnectionPool rewrite and http/2 support #376).

[glbrntt]

Looks good aside from a typo.

[glbrntt]

typo: expectedAuthroization -> expectedAuthorization

[fabianfett]

Fixed!

[Davidde94]

@swift-server-bot test this please