Add timestampts to JSON logs

[goodlyrottenapple]

Fixes #3894

[goodlyrottenapple]

There is now no way (I think) to filter the message once we already start converting it to LogStr, so we would either have to render it twice, once in the filter (where we could then check if it is empty/null) and second time when we actually want to log it. As this would be inefficient, I've added a type-class function which can be used to control if some kore Entry is just used for context and should thus not be emitted as a message. See occurrences of isEmpty in this PR to see what I mean.

[geo2a]

Nice! I've played around with both booster-dev and kore-rpc-booster and the logging options. The JSON timestamp work as I'd expect.

[jberthold]

LGTM (haven't tested it, though) tested now

[jberthold]

While at it, could you please remove the dot from https://github.com/runtimeverification/haskell-backend/pull/3925/files#diff-fdf134fb9c13c1143cd1c9d16f037c810a3f29d61d40c9944b898af886bb61e9R188 so that we can parse the timestamps later?

    let formatString = "%Y-%m-%dT%H:%M:%S%6Q"

The %Q includes its own decimal point if the value is not zero , see https://hackage.haskell.org/package/time-1.14/docs/Data-Time-Format.html