Improve 'Custom transports' docs

[lovelydinosaur]

Now includes examples with...

• HelloWorldTransport
• HTTPSRedirect
• DebuggingTransport
• ProxyRoundRobin

[karpetrosyan]

One more useful example:

import httpx


class FirstMiddleWareTransport(httpx.BaseTransport):
    def __init__(self, next_transport: httpx.BaseTransport):
        self.next_transport = next_transport

    def handle_request(self, request: httpx.Request) -> httpx.Response:
        print("First middleware request")
        response = self.next_transport.handle_request(request)
        print("First middleware response")
        return response


class SecondMiddleWareTransport(httpx.BaseTransport):
    def __init__(self, next_transport: httpx.BaseTransport):
        self.next_transport = next_transport

    def handle_request(self, request: httpx.Request) -> httpx.Response:
        print("Second middleware request")
        response = self.next_transport.handle_request(request)
        print("Second middleware response")
        return response


middleware_transport = FirstMiddleWareTransport(
    next_transport=SecondMiddleWareTransport(next_transport=httpx.HTTPTransport())
)
client = httpx.Client(transport=middleware_transport)

client.get("https://www.encode.io")

OUTPUT

First middleware request
Second middleware request
Second middleware response
First middleware response

[lovelydinosaur]

@karpetrosyan I like the intent of demonstrating how transports can be used as nestable middleware;
I'm less keen on an abstract example that's not demo'ing a real-life use-case.

(Should we drop the "hello world" example, or is the PR okay as it currently stands?)

[karpetrosyan]

(Should we drop the "hello world" example, or is the PR okay as it currently stands?)

It's up to you, it looks great anyway.

[lovelydinosaur]

Thanks @karpetrosyan appreciate your time. 🙏🏼