Add riverlog.LoggerSafely for access outside work body without panic

[brandur]

Here, add a "safe" variant of riverlog.Logger: rigerlog.LoggerSafely.

While it'd be rare to not know whether you had a logger or not from
inside a Work function, Work functions might call into other Go
modules in your project, those modules might be reused in non-Work
contexts, and it might not always be clear whether a logger should be
available or not.

This function lets users do a conditional check before logging:

if logger, ok := riverlog.LoggerSafely(ctx); ok {
    logger.InfoContext(ctx, "Hello from logger")
}

I went with the "safely" naming according to our conventions elsewhere,
but one thing I'm not sure about is that ClientFromContextSafely which
is a similar function, returns an error instead of ok boolean. Here we
break with that convention somewhat, but I think the boolean version is
more ergonomic. Another option might be to give this a slightly
alternative naming convention like LoggerOK or LoggerMaybe.

Fixes #1092.

[brandur]

The other option is that we could just get rid of the panic and have Logger return a simple nil in case it wasn't available in context. Not sure which version is better, although that one would be a little more risky in causing nil pointer exceptions.

[bgentry]

This seems like a good solution 👍 Thanks for getting up a fix quickly 🙏

[brandur]

Woot. Thx!