feat: implement credential offer URI dereferencing

## Summary

Implement HTTP fetch to dereference `credential_offer_uri` URLs for credential offers passed by reference.

## Background

The `CredentialOfferSource` enum supports two variants:
- `ByValue(CredentialOffer)` - Already parsed credential offer
- `ByReference(String)` - URI that needs to be fetched

Currently, only the `ByValue` path is fully implemented. We need to implement the HTTP fetch to dereference `ByReference` URIs.

## Requirements

- Add HTTP client dependency (e.g., `reqwest` or use existing)
- Implement `resolve()` method on `CredentialOfferUri` that:
- Returns the `CredentialOffer` directly for `ByValue`
- Fetches and parses the JSON for `ByReference`
- Handle errors appropriately (network errors, invalid JSON, etc.)
- Add tests for URI dereferencing

## Related

- PR #77 - Credential offer data models
- OpenID4VCI Section 4.1

## Milestone

M1