Proposal: hyper `Flag` service

With all of our customers, we advocate for trunk based development, and separating release from deployment; code ought to be continuously delivered, with features being shipped behind flags. The DORA metrics show that this is what high performing teams _do_.

Feature flags are so essential to this way of delivering, that it would make sense for the service framework to have a `Flag` service. The api would be really simple, basically a key-value store, but geared towards feature flags:

```ts
// Store api
create(name: string): Promise<{ ok: boolean }>
destroy(name: string): Promise<{ ok: boolean }>
// Flag api
get (name: string): Promise<{ ok: boolean, flag: any }>
set (name: string, value: any): Promise<{ ok: boolean }>
remove(name: string): Promise<{ ok: boolean }>
```

Since flags can be more than just booleans, it makes sense to allow the value to be anything. Maybe there are other apis, but this is simple, and general purpose, in the spirit of the framework.

This API could be implemented on _so many_ technologies -- practically anything that can store key-value pairs. A sqlite adapter would be enough for most teams. Adapters could also be written on top of SaaS like LaunchDarkly, but starting out that'd most likely be overkill.

The issue proposes a new Flag service but names no implementation files, entry points, or tests. Start by locating the service interfaces and adapter structure in the repository, then determine how a general-purpose flag API and a SQLite adapter would fit them. Done would require an agreed API, implementation approach, and corresponding tests.

Scritto dal modello di indicizzazione a partire dal testo della issue.