feat(slim-bindings): hierarchical config loading and stable identity

[Tehsmash]

Description

This PR ports the hierarchical SLIM configuration loading feature from the slim-bindings PR #21 into the SLIM monorepo, and then lifts the core logic into agntcy-slim-service so any Rust consumer can use it without the UniFFI layer.

Commit 1 — feat(slim-bindings): hierarchical config loading and stable identity

Adds crates/slim-bindings/src/slim_node_config.rs and extends crates/slim-bindings/src/service.rs with:

• SlimConfig / SlimAppConfig UniFFI record types
• load_slim_config() — git-style slim.yaml discovery + env-var overrides
• Service::create_app_from_slim_config — one-call API: load config → init identity → connect → subscribe → return SlimAppHandle
• Signature key caching in .slim-cache/ for stable agent identity across restarts

Commit 2 — feat(service): add slim-config feature with hierarchical config loading

Moves the config-loading logic into agntcy-slim-service behind a new slim-config Cargo feature, so downstream consumers using core crates directly can benefit:

• New crates/service/src/node_config.rs module using core types (slim_config::client::ClientConfig, slim_config::auth::identity::*)
• Service::create_app_from_slim_config(&self, config: SlimNodeConfig) -> Result<AppHandle, ServiceError> (gated on slim-config + session)
• AppHandle struct bundling app, notification receiver, name, and conn_id
• #[serde(default)] added to IdentityProviderConfig::SharedSecret::id so the field is optional in YAML
• slim-bindings refactored to delegate to the service crate — load_slim_config and create_app_from_slim_config are now thin FFI adapters

Type of Change

• New Feature
• Refactor

Checklist

• I have read the contributing guidelines
• Existing issues have been referenced (where applicable)
• I have verified this change is not present in other open pull requests
• Functionality is documented
• All code style checks pass
• New code contribution is covered by automated tests
• All new and existing tests pass

[micpapal]

once #1820 is merged I think we can remove the slim-cache file as the keys are already store in the DB. We can add a new filed in slim.yaml that should look like

node:
  endpoint: "http://localhost:46357"

app:
  name: "agntcy/default/receiver"
  identity:
    type: shared_secret
    data: "slim-dev-shared-secret-change-in-production"
  identity_verifier:
    type: shared_secret
    data: "slim-dev-shared-secret-change-in-production"

persistence:
  path: "~/.slim/state"        # where the SQLite DB lives
  # passphrase: "..."          # optional, for real at-rest encryption

using create_app_with_direction_and_persistence we should be able to recreate the app and all the related session.