Add HTTP Cache Override Support

[posborne]

Overview

Add support for Fastly's HTTP Cache API, providing HTTP-aware caching with request/response handling and RFC 9111 compliance helpers.

WIT Interface

The WIT interface for http-cache is large, not included inline here.

API Design

• Implement HttpCacheEntry resource wrapper for HTTP-aware cache operations
• Provide HTTP Request/Response integration (wraps core cache with HTTP types)
• Helper functions: is_request_cacheable(), suggested options generators
• Support RFC 9111 compliance patterns
• Cache override capabilities (TTL, stale-while-revalidate, surrogate keys)

Cross-SDK Comparison: Rust uses Request::set_cache_override() with enum variants (None/Pass/Override). Go and JS integrate cache override into request configuration. Python should provide similar enum-based API (CacheOverride.none(), .pass_(), .override(ttl=..., surrogate_key=...)).

Viceroy Testing

Viceroy's HTTP cache implementation follows similar limitations to Core Cache. Testing focus should be on:

• Verifying cache override API calls work correctly
• Testing SDK wrapper logic (option parsing, TTL handling, surrogate key formatting)
• Ensuring HTTP cacheability checks function properly
• Validating integration with Request/Response objects

Do NOT:

• Try to verify actual cache behavior or persistence
• Create elaborate tests around Viceroy's stubbed caching
• Test cache hit/miss scenarios (this is host behavior, not SDK code)

The goal is to verify our Python wrapper code works, not to test Viceroy's cache implementation.

Reference

• Fastly HTTP Cache Documentation
• RFC 9111 HTTP Caching