Investigate WIT interface version upgrades

[lann]

How do we handle backward-compatible (i.e. adding a function) WIT interface upgrades?

[itowlson]

This is a consideration for adding date/time types to rdbms-types which it would be good to progress for Spin 3.

I'm not sure if the package/world revs to add wasi:config and wasi:keyvalue also fall under this, but if not, well, we need to figure out how to deliver those too... grin