Worker heartbeat

Is your feature request related to a problem? Please describe.

An issue that has come up a few times in the community slack is auto-activity heartbeats.
A lot of the time heartbeats are used just to verify that the activity is still running on a Worker or there might not be a simple way to heartbeat from the activity logic in a timely manner so heartbeats are implemented "externally" as follows:

async function autoheartbeat(fn) {
  async function heartbeat() {
    const cx = activity.Context.current();
    for (;;) {
      await cx.sleep(cx.info.heartbeatTimeout / 2);
      cx.heartbeat();
    }
  }
  return (...args) => Promise.race([heartbeat(), fn(...args)]);
}

export const myActivity = autoheartbeat(async (arg1, arg2) => {
  // activity logic
});

Describe the solution you'd like

Add support for Worker heartbeat so activities automatically failover to a new Worker if the current executing Worker goes down.

Describe alternatives you've considered

A trivial alternative is to ask users to wrap their activity function in an "autoheartbeat" wrapper as shown above or add SDK support for auto activity heartbeat.

Per-activity heartbeat is noisier than Worker heartbeat so it'd be better if we supported Worker heartbeats.

Start with the Worker heartbeat proposal and the existing per-activity heartbeat behavior described in the issue. Trace how a Worker currently reports activity liveness and how failover is triggered. Done means activities can fail over when their executing Worker goes down without requiring users to add an autoheartbeat wrapper.