Implement prop#269 "Hybrid Handshakes" (ntor2 module)
As part of #24985 (moved), we'll need to implement prop#269. This should probably be in two parts:
1) Implement a new module which, given function pointers to two handshakes (one classic and the other post-quantum secure) which follow some prescribed API, compose the handshakes together to produce a final shared secret
2) Implement "hybrid null" (as it's called in the proposal), which here I'm calling "ntor2"This ticket is about part 2. It ultimately depends upon #24986 (moved) and #24987 (moved) as well, although it can be done (and even rolled out into production, if we chose to do so) before either are finished.
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