PEP 807: make discovery procedure reversible

[woodruffw]

This is my last intended change to PEP 807 before submitting to another round of review.

What I've done here is make the discovery flow (which was already defined in the PEP) reversible: the previous flow used a strong trapdoor function (SHA-256) to compute the "discovery key," which effectively meant that indices would need to precompute every valid discovery key. This quickly becomes infeasible (or at least very annoying) for index topologies with ~thousands of distinct upload endpoints, particularly if those endpoints can vary by case, escaping, etc.

The alternative is much simpler: instead of using a trapdoor function, we compute the discovery key as a query-safe URL encoding. This is completely reversible, less opaque, and is equally built into Python (with quote_plus).

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