feat(registry): add mDOC IACA trust registry

Implements TrustRegistry for mDOC IACA certificate validation:
- Fetches IACAs dynamically from issuer's mdoc_iacas_uri endpoint
- Validates X5C chains against fetched IACAs
- Supports issuer allowlist policy enforcement
- Caches IACAs with configurable TTL
- Discovers metadata via OpenID4VCI .well-known endpoint

Includes comprehensive test suite (23 tests) with:
- Mock HTTP server for issuer simulation
- Certificate generation helpers for test chains
- Cache behavior and refresh tests
- Error handling and edge case coverage

Related to ADR-012 in go-wallet-backend.

Author: leifj