Initialization Tasks (e.g. creating database / seeding data)

Developers can use the --sqlite flag as part of spin up and provide SQL commands to perform common tasks like executing DDL and/or DML before the actual Spin App bootstraps. Although this is great and useful for some situations, there are currently limitations:

• Only available for SQLite
• Only affects the default database

Facing those limitations results in developers coming up with different patterns, layouts, scripts, or make targets to bootstrap databases before running their Spin App. I think we could improve Spin here and make it more holistic to provide a general purpose initialization stage.

From DX point of view I would love to have a unified experience to perform initialization logic. Although I had different potential solutions in my mind, I wanna share my personal favorite approach here:

Wouldn't it be possible to provide an additional macro and let developers "code" their initialization logic? I'm thinking about something like shown below. 👇🏻

#[initialize]
pub fn initialize() -> anyhow::Result<()> {
  // perform initialization logic
}

#[http_component]
pub fn handle_req(req: Request) -> anyhow::Result<impl IntoResponse> {
  Ok(Response::new(200,())
}

This would allow developers to use variables, and environment variables configured in their spin.toml and to run initialization logic - like database seeding, no matter which DBMS they use.

The issue names no files or tests; start by tracing the existing spin up --sqlite initialization path and how Spin apps bootstrap. Compare that behavior with the proposed #[initialize] entry point, then define the supported initialization scope and verify that the resulting design works beyond SQLite and the default database.