Upgrade payjoin bindings to use payjoin_ffi

[benalleng]

With the newest payjoin bindings becoming stabilized around the latest payjoin 1.0 rc and 0.25 it is a good time to upgrade away from payjoin_flutter.

• update flutter to use a dart sdk >=3.10.0 to accept native_toolchain_rust to be able to build the payjoin binary for the new ffi bindings.

• replace payjoin_flutter in favor of payjoin_ffi and upgrade the API to the latest 0.25/1.0.2-rc release

• test against payjoin-cli to prove out the implementation

[benalleng]

Upon first pass it seems the primary blocker for an upgrade is the hive project in general, it is just unfortunately abandoned and hive_generator in particular is no longer compatible with a dart sdk >=3.10.0

[MrCyjaneK]

@benalleng this is an issue that we are slowly fixing by migrating over to SQLite, though this will take some time

[DanGould]

@chavic can you check if it's possible for uniffi-dart to downgrade to 3.8, and if not, what the fundamental constraint is

[chavic]

@DanGould @benalleng I pushed a branch that backs out the native-assets packaging path and restores the old manual Dart library loading flow. This was the main blocker to downgrading, chavic/investigate-dart-3-8.. This should unblock the generation path for now.

[DanGould]

@chavic if this is workable and we don't' need any features from dart > 3.10 I think it should be PR'd so that we can be compatible with Cake on our payjoin dart release.