Improve caching for status list VCs and other documents generally

[dlongley]

Status list VCs should be cached, even if HTTP headers are not set to instruct the client what to do. Additionally, node.js doesn't have a built-in HTTP client cache at this time. There are some tutorials for integrating HTTP client caching with undici, but even if we could change the global dispatcher to capture responses for caching or similar, that might be too intrusive a pattern here.

We should figure out how to specifically make the document loader passed to do status checking cache SL VCs.