feat(frontend): introducing pow worker for allow_signing protection

[DecentAgeCoder]

Motivation

This PR introduces new functionality to handle proof-of-work (PoW) challenges for calling allow_signing endpoint

Changes

• Introduced a short polling (request - repspone) post message design for message communication between the pow.worker.ts and worker.pow.services.ts
• Added Other as a new variant for errors in CreateChallengeError across several files.
• Added new functions for managing proof-of-work challenges: createPowChallenge and allowSigningResult.
• Implemented new PowProtector Service and PoW worker functionalities.
• Integrated PowProtector into the frontend loader component.
• Added cryptographic hashing utilities and worker management helpers to support PoW.
• Updated schemas and types for backend communication, ensuring strong type safety in PoW-related operations.

Tests

[AntonioVentilii]

Let's split this PR

• the backend canister API
• the utils
• the worker
• the Svelte component