Implement Cacher Interface and FileCacher Structure

[piraz]

Establish a dedicated cache package to decouple storage logic from the protocol. This includes a generic Cacher interface to allow for future storage implementations (e.g., memory or database) and a FileCacher for local filesystem persistence.

Tasks

• Create internal/cache/cacher.go.
• Define the Cacher interface with Get, Set, and IsExpired methods.
• Implement the FileCacher struct and its logic.
• Ensure thread-safe directory creation and file permissions.

Acceptance Criteria

• The cache package is independent of the protocol package.
• FileCacher correctly persists strings to the disk.
• TTL is respected when checking for cache validity.