prop224: Add a trunnel subdirectory specifically for HS
We'll be using trunnel for all cells parsing in proposal 224. This ticket is to add a subdirectory in src/trunnel/hs that will contain one .trunnel file per cell and most likely a common file for shared binary structure.
This is the starting point for #17241 (moved) so if we can get this upstream as soon as possible, it makes thing much more easier to split the work between developers and branches instead of carrying a commit that adds that directory to the build system.
The plan is to add two cell definition at first ESTABLISH_INTRO and INTRODUCE1 and then add more as we implement them.
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