Support fetching intermediates

[ctz]

• massively untested
• needs a new abstraction over index building at the end of fetching (index building for revocation, openssl symlink farming for intermediates)

[codspeed-hq]

Merging this PR will not alter performance

✅ 4 untouched benchmarks

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_{Comparing jbp-intermediate-fetch (c9a5dc8) with main (9655fd7)}

[ctz]

(periodic rebase, nothing to see here)