TimespanLogging: Provide log serialization helper

It's super common to want to export logs to a file to be later imported for analysis. We could provide an interface to export to any Tables.jl-compatible sink to make this easy.

An open question is how to serialize streams of a `MultiEventLog`, which tend to have different formats. These also often contain data like `Dict` or `NamedTuple` which don't serialize nicely as column values in CSV; maybe we should also have a JSON-based format? If we emit JSON, we could also move most of the sanitization code from DaggerWebDash over to TimespanLogging.