Support templating in variables

[rylev]

Variable support was added in #19, but templating is not supported. To do this, we should use spin-expressions (which is luckily already a dependency of spin-test-virt). Unfortunately, however, resolving a key can only be done asynchronously. This method is async because of variable providers being async. However, we won't be using variable providers, so in theory there can easily be a sync version of this method exposed that we can use.

[rylev]

I have a fix for this based on fermyon/spin#2458.