Add hooks for socket configuration

[John-Connolly]

Description

Some user may need more advanced socket options for Vapors HTTP server. There is currently no way of specifying these options. This PR solves the issue by adding hooks for the server bootstrap. Many socket options such as SO_REUSEPORT need to be set before the server is started. Providing users with a configuration handler seems like a logical way of accomplishing this.

Example usage

 app.http.server.configuration.serverChannelOptionConfiguration = { serverBootstrap in
        serverBootstrap.serverChannelOption(ChannelOptions.socket(SocketOptionLevel(SOL_SOCKET), SO_REUSEPORT), value: SocketOptionValue(1))
 }

    app.http.server.configuration.childChannelOptionConfiguration = { serverBootstrap in
        serverBootstrap.childChannelOption(ChannelOptions.socket(SocketOptionLevel(SOL_SOCKET), SO_REUSEPORT), value: SocketOptionValue(1))
 }

[Joannis]

I really don't like adding this niche use case to the initializer

[Joannis]

I think it's a good addition, but I dislike the way it clutters the initializer for such a niche use case.

[John-Connolly]

I think it's a good addition, but I dislike the way it clutters the initializer for such a niche use case.

@Joannis Thanks for the feedback. I tend to agree, 90 % of vapor users will not have a need to use this option. Thoughts on this being more akin to the customCertificateVerifyCallback that way the initializer doesn't need to change.