feat(`anvil`): be able to specify a prefix path for temp disk storage

[kahuang]

Component

Anvil

Describe the feature you would like

When anvil state grows, it persists it temporarily to disk. I'd like to be able to specify a prefix path for the directory that gets created by anvil for this state.

Additional context

I am running Anvil in the cloud, and want to attach a persistent disk to the instance and mount it at the path anvil will use for storing state. Because it's a random directory each time, there's no way for me to programatically mount my disk at that location before anvil starts.