tor-cell: Add encodable conflux relay message types.

gabi-250requested to merge
gabi-250/arti:conflux-relay-cmd into main

This adds support for encoding and decoding RELAY_CONFLUX_LINK, RELAY_CONFLUX_LINKED, and RELAY_CONFLUX_SWITCH cells.

cc @opara @dgoulet

Closes #1852 (closed)

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