Storing flows

[jgraef]

We might want to store flows for later inspection. My idea was to use sqlx with sqlite for this. sqlite allows us to write complex queries that are optimally executed and to store everything in one file. sqlx is nice because it verifies SQL queries at compile-time.

[jgraef]

get/insert for flows/messages is implemented. We might want to add code for artifacts later.

This currently uses sqlx's built-in migration feature, but we might want to roll our own.

I will close this for now.