Spin config/secret store implementation

[aram356]

User Story

As a developer deploying to Fermyon Spin, I want to use the EdgeZero config store and secret store APIs, so that I can read application configuration and secrets from my Spin application using the same interface as Fastly, Cloudflare, and Axum adapters.

Background

The ConfigStore and SecretStore traits are defined in edgezero-core. Fastly, Cloudflare, and Axum adapters implement both traits. The Spin adapter currently has no store support. Spin exposes configuration via spin_sdk::variables and secrets via the same variables API with protected values.

Acceptance Criteria

• SpinConfigStore implements ConfigStore from edgezero-core using the spin-sdk variables API
• SpinSecretStore implements SecretStore from edgezero-core using the spin-sdk variables API (secrets are protected variables)
• Both types are exported from edgezero-adapter-spin
• Manifest validation in edgezero-core recognises spin as a supported config store and secret store adapter
• Contract tests pass for both SpinConfigStore and SpinSecretStore
• No Tokio or native-only dependencies introduced (WASM-compatible only)
• cargo clippy and cargo fmt pass

Affected Area

Adapter -- Spin