feat: add annotations to subscriptions table

[yordis]

Context

The intent is to add information to build Operational dashboards based on such metadata. I need to contact the team owner of such a processor as soon as possible.

• Should this be outside the Event Store?

This is a way to avoid extra infrastructure and tooling, and the code should be the source of truth rather than conforming to more complex organizational structures and more prominent companies' needs.

Here is an example:

defmodule ExampleHandler do
  use Commanded.Event.Handler,
    application: ExampleApp,
    name: "ExampleHandler",
    annotations: %{team: "billing"} # made up, pending to discuss in Commanded itself
end

Open Questions

• Should the migration allow NULL values?
• Should we do a merge when deploying or replace annotations when subscribing?

[yordis]

I am confused about this file btw, do we even need these files since we use the SQL init module?