Prototype of intermediate preloading

[ctz]

This is the mirroring-side piece of this. It establishes that the mirror manifest names follow this format:

• /revocation/manifest.json -- the manifest fetched for current revocation filters (plain /manifest.json retained)
• /intermediates/manifest.json - the manifest fetched for intermediates, introduced in this PR.

[codspeed-hq]

Merging this PR will not alter performance

✅ 4 untouched benchmarks

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_{Comparing jbp-intermediates (66bf228) with main (5f8b57b)}