Pad INTRODUCE cells to hide the number of link specifiers
Before #23577 (moved), there are two sizes of introduce cell data:
- legacy id and IPv4 (24 bytes)
- legacy id and IPv4 and ed25519 id (56 bytes)
After #23577 (moved): there are another two sizes:
- legacy id and IPv4 and IPv6 (40 bytes)
- legacy id and IPv4 and ed25519 id and IPv6 (72 bytes)
And if we ever remove IPv4 addresses, or add another link specifier, we create even more sizes.
This is ok at the moment, because we pad all cells to HS_CELL_INTRODUCE1_MIN_SIZE (246 bytes), and no cells are larger than this size. But we should make sure we are padding larger cells to the maximum cell length, to avoid splitting the rendezvous point anonymity set with future link specifiers.
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