Add a timeout getter to HandlerContext and export createHandlerContext

[timostamm]

Following #591, this adds a method timeoutMs() to the handler context for true deadline propagation:

import type { HandlerContext } from "@bufbuild/connect";

const say = async (req: SayRequest, ctx: HandlerContext) => {
  
  // If a timeout was set on the call to this service, the timeout() method 
  // returns the remaining time in milliseconds. 
  // Passing the value to an upstream client call propagates the timeout.
  await client.someCall({}, { timeoutMs: ctx.timeoutMs() });

  return new SayResponse({sentence: `You said: ${req.sentence}`});
};

This is an alternative to passing ctx.signal to upstream client calls. Passing ctx.signal works for any operation you might want to call (as long as the API supports it). For RPC calls, passing the timeout is preferable because it is more efficient and robust - you have a guarantee that the peer is aware of the deadline, regardless of network issues.

We now also export createHandlerContext() from @bufbuild/connect. If you want to run unit tests against your service implementations, you now have a convenient function to create a HandlerContext if your services rely on it.

This PR also enables the test case cancel_after_first_response, refactors the code for deadlines so that it can be used on the client-side in a follow-up, and adds more test coverage