Add codepoint obfuscation via grouping to the patch subset specification.

[garretrieger]

This adds a grouping mechanism to the patch subset client specification. This requires that for certain blocks of codepoints those codepoints always be requested in groups with a minimum size.

The impact of grouping on privacy was simulated here: https://github.com/w3c/PFE-analysis/blob/main/results/noise_simulations.md

There's still a couple of open questions we probably want to discuss with the WG before merging this:

1. What is an appropriate minimum group size? for now I've chosen 7 based on the simulation results.
2. Which codepoints specifically should we add to the list of codepoints to be grouped? Currently I only include all of the CJK ideograph blocks, but not any of the other CJK blocks (eg. punctuation, compatibility points, and so on)

Addresses: #42 and #50

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[vlevantovsky]

Looks good, thanks!