Implement the `multi` provider flag

Implement some form of `multi` providers. Is a flag on the provider the best way?

How to allow multiple providers while still allowing proper overwriting with single injected providers?

The issue is about adding a 'multi' provider flag to the dependency injection framework. Start by examining the core provider registration and resolution logic in the codebase. Look for existing provider types and how overwriting works. Understanding the framework's architecture for handling single vs. multiple providers is key. The goal is to design a system that allows multiple providers for a token while respecting single provider overrides.

Written by the indexing model from the issue text.