Feature: Low-level hooks / custom connectors

[Simon Bernier St-Pierre]

Initially suggested by @adamreichold here.

Generally, allow people to hook into the logic of the library to customize the behavior. A few places where this might be useful:

• Initializing TCP connection
• Initializing TLS
• Handling redirection responses
• Providing a custom stream implementation that implements a request timeout
• Tracing and debugging

[Adam Reichold]

@sbstp I would prefer if we closed this as you made your API design choice clear and I fear this here would run counter to it. I think it is best if we try to do one way well instead of making things too complicated by providing every possible way. (The only thing here I still as useful would be to pass a preconfigured TlsConnector to handle things like client certificates to avoid having to basically provide wrapper for all of TlsConnectorBuilder.)