Add the API to modify the time, for time syncing

[sunfishcode]

On typical platforms, regular applications don't have permission to set the time, so there are very few applications and libraries which do this in practice. Consequently, it seems like the use cases for this would be limited. It seems best for WASI clocks to focus on just reading the time for now, and if an API for setting the time is needed in the future, that can be added as a separate API.

[sunfishcode]

Closing, so that wasi-clocks can focus on just reading clocks for now. We can revisit setting clocks in future iterations.