Add Config.SoftStopTimeout for a cleaner way to shut a client down

[brandur]

A part of River code that's always bothered me is the graceful shutdown
example [1]. It's really, really complicated, and I think that
complicated orchestration is very likely a sign that our code isn't
quite right.

I initially tried putting the graceful shutdown code into a helper like
river.GracefulShutdown, but I found that also problematic because it's
not super clean to call, and even minor customizations would require a
user to copy out the entire function.

Playing with an LLM a bit, I came up with this alternative: build an
understanding of soft/hard stop into the client's default Stop
function by adding Config.SoftStopTimeout. By default, leaving this
value unconfigured keeps the existing behavior of River today where a
call to Stop waits unbounded time for jobs to finish working. Adding a
SoftStopTimeout value brings in functionality similar to the graceful
shutdown example: stopping proceeds normally waiting for jobs to finish,
but in case they don't inside of SoftStopTimeout, they're cancelled as
if StopAndCancel had been called.

The soft timeout is also respected for a cancelled Start context,
which makes the basic use of River for soft/hard stop very nice (you
don't even need to call Stop anymore):

// Use signal.NotifyContext to cancel the start context on SIGINT/SIGTERM.
// When the signal fires, the client initiates a soft stop. If running jobs
// don't finish within SoftStopTimeout, their contexts are automatically
// cancelled.
ctx, stop := signal.NotifyContext(ctx, syscall.SIGINT, syscall.SIGTERM)
defer stop()

if err := riverClient.Start(ctx); err != nil {
    panic(err)
}

[1] https://github.com/riverqueue/river/blob/master/example_graceful_shutdown_test.go