To make it easier to integrate without losing flexibility, this commit brings a pre-built Store implementation using ETS. The downside is that it's in-memory and any restart loses in-progress requests as well as any existing access tokens. In an app this would be experienced as getting logged out early. Not ideal, but also not the worst. For an implementation that survives restarts, take a look at annot.at's Store implementation here https://tangled.org/jola.dev/annot.at/blob/c42010e2b62972e445de4a3a9c89c99155fab4ff/lib/annot_at/latch_store.ex