Consider moving `multiparty` out of master and into its own branch

[DanGould]

As shown in the following code, the underlying v1/v2 architecture is changing rapidly, and multiparty has accumulated so much tech debt that it's a burden on new moves. I suggest we move it to a branch for now, since nobody is running it in production, and re-introduce it once we figure out a more stable v1/v2 abstraction in our architecture.

The following single struct is not a struct implementing the State trait but a full on struct, and it seems that that would need to be refactored into a proper multiparty::Sender::<State> pattern in order to be maintainable and congruent with v2.

rust-payjoin/payjoin/src/core/send/multiparty/mod.rs

Line 121 in a5d82d1

pub struct GetContext(v2::Sender<v2::V2GetContext>);

[arminsabouri]

I agree. Its been a real slog trying to maintain this thing while adding SEL and updating the rest of the API. The only thing I will add is that when we ressurect it it should also include:

• persistance via session events

[spacebear21]

Closed by #923