WIP - encrypted fragments bip outline

[arminsabouri]

Original description :

Super rough outline of what I think this document should cover. It may be the case that in writing this document we realize there is a much better organization or outline. Many of open questions in the comments.

The rest of the work on this document can be parallelized.

The rest of the psbt specific details such as what goes into keydata and valuedata. And what are the practical and privacy implications / tradeoffs.
Specify the rest of the HPKE details.
Specify privacy considerations (paddings, bip234 elligatorswift)
I personally sometimes need to work backwards on this type of stuff. i.e write the code and/or flow diagrams and then a spec becomes more apparent. But eventually we will need a reference and some test vectors
Please comment in this thread what parts you want to focus on @zealsham @Mshehu5 @0xZaddyy

[nothingmuch]

https://github.com/FiloSottile/hpke/blob/main/hpke-pq.md