Improve middleware situation

[Firehed]

• PSR-15
• Routing process could possibly become MW
• Input validation, auth, and error handling same?

The goal is that the auth and error handling could move out of the main EndpointInterface, and auth moves into some kind of AuthenticatedEndpointInterface in 4.0, so changes should enable this. Error handling may get its own interface; probably inspired by MiddlewareInterface but also taking Throwable in place of the ResponseHandlerInterface

For auth specifically, it will need access to the endpoint being authenticated against. The attributes component of ServerRequestInterface is a possibility here, but that's going to be really challenging to get things in the right order (and other MW could strip it out or otherwise make a mess). PSR-15 meta goes into this a bit. Alternatively, extending the MiddlewareInterface to have an Endpoint injector may work

[Firehed]

All of the auth stuff previously mentioned is largely redundant with #43, so trying to shoehorn something in with request attributes isn't necessary.

Everything else is still fairly open ended, but I think it looks like this:

• Dispatcher becomes a RequestHandlerInterface, doing the FIFO queue thing (return $middleware->process($req, $this))
• Some kind of adapter for legacy MW is created, and they are added to the end of the queue when dispatch is run