LibWeb: HTTP caching

[awesomekling]

We should implement a HTTP cache and hook it up to the Fetch implementation in LibWeb.

Fetch spec: https://fetch.spec.whatwg.org/
HTTP caching spec: https://httpwg.org/specs/rfc9111.html

Files should be stored to disk so they persist across browser restarts.

See specifically https://fetch.spec.whatwg.org/#concept-http-network-or-cache-fetch and our (absence of) implementation in Web::Fetch::Fetching::http_network_or_cache_fetch():

    // 6. Let httpCache be null.
    // (Typeless until we actually implement it, needed for checks below)
    void* http_cache = nullptr;

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