feat: edfs refactor kafka and nats with a datasource per field

[alepane21]

Motivation and Context

Today changing or adding an EDFS provider requires changes to cosmo and the engine, making it harder than it should.
The main point of this PR is to make it easier to add future EDFS providers.

So I moved all the pubsub code from the engine to the pubsub package in the router and refactored it to make more sense with the new structure.

There is a README that will help implementing new EDFS providers (manually or using AI).

Important points

• I added a test to verify that kafka publish is working, there was nothing testing it before;
• connections operations (connection, closing connection) should happen as before;
• switching from a previous version to the new one should just works, without any change to the configuration or schema.

Design

Interface	Responsibility
ProviderBuilder	Allows the router to build a Provider starting from DataSourceConfiguration and router event config.
Provider	Owns provider‑wide resources for a single providerId (e.g. NATS connection pool).
EngineDataSourceFactory	Implements ResolveDataSource + SubscribeDataSource for one GraphQL field.
Adapter	Thin, protocol‑specific driver that actually publishes / subscribes.

Execution flow

• When the router’s Loader (router/core/factoryresolver.go) encounters a PubSub nodes, it collects them and build the data sources and PubSubProviders by passing all the collected datasources to the PubSubProviderBuilder
• The PubSubProviderBuilder will create a single PubSubDataSource for each root node with its adapter and event data already assigned
  • this will generate more data sources, but will reduce the operations to do after the schema has been loaded
• The adapter will hands off the actual publish / subscribe work to the appropriate external driver (Kafka, NATS, etc.).

Checklist

• I have discussed my proposed changes in an issue and have received approval to proceed.
• I have followed the coding standards of the project.
• Tests or benchmarks have been added or updated.
• Documentation has been updated on https://github.com/wundergraph/cosmo-docs.
• I have read the Contributors Guide.

[devsergiy]

Great Job!