Bring `payjoin-ffi` into top level workspace

[DanGould]

payjoin-ffi was brought into this repo outside of the main workspace as a first step. The next step is to get it into the workspace so that it directly depends on payjoin from path (patched in the workspace) rather than 0.23 directly.

This ought to happen sooner than later so that future changes can be made throughout the whole stack at once to see their consequences at depth.

See #719 for the confusion this is causing before payjoin and payjoin-ffi are synced up

[DanGould]

@0xBEEFCAF3 we may have a conflict here where payjoin-ffi depends on "uniffi/cli" feature brings in clap "4" but payjoin-cli depends on clap v4.0.32 in order to be 1.63 msrv

[spacebear21]

I ran into that exact issue with clap trying to bring payjoin-ffi into the main workspace. I think payjoin-ffi would need to conform to MSRV for this to work.

[DanGould]

This issue was misguided. I had previously thought we weren't patching the local ../payjoin into payjoin-ffi, and would need to bring it into the same workspace to do so. however, it seems that the current tip at 16f0c20 already patches ../payjoin in. So in order to keep MSRV separate, we can leave the separate Cargo.toml workspaces and everything should work fine. We don't need to fix this issue.

[DanGould]

Oh, but we should make payjoin-ffi build and test some complete test suite if any of its files are changed, not just if payjoin-ffi/python changes, as @0xBEEFCAF3 mentioned on call.