Uniffi dart integration test (#3)

* Initialize payjoin_dart

* Add initial dart tests to prove out the utilization of uniffi-dart

Minimizing the import I was able to get rid of all errors introduced by
uniffi-dart and built a successful uniffi binding.

These tests are just the barebones of what is built but proves out that
we have some functioning bindings when creating a minimal build.

* Add persistence unit tests

* Specify the language to create bindings at the bindings creation step

I've included support for including --language args beyond those explicitly
supported in uniffi-rs with a match block that passes the chosen language
to chose what binding cargo package to use.

* Add dart bindings generation scripts

* Add dart github workflow

* [WIP] integration tests

* [WIP] almost done

* determine proposal return type

* fix type defs

* spellcheck

* add http dep

* fix Record constructors

The constructors are public as of uniffi-dart db4eed7

* Fix directory arg

the FFI create_session method takes a String, not a URL.

* remove null placeholders

uniffi-dart empty sequence handling was fixed in 8536f95

* fix some null pointers

* state machine function return types

* use asString() for URLs

toString() isn't defined due to Display trait not yet implemented in
uniffi-dart, so use asString() instead which is defined.

* Fix build_sweep_psbt

Pass the "psbt" value as parameter, and return a String instead of a
bitcoin.Psbt

* s/Uri.https/Uri/parse

Uri.https() expects only the host portion, not a complete URL. Use
Uri.parse() instead.

* Fix process_receiver_proposal

* Fix response bytes

* Fix RPC calls

* Fix test assertions

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Co-authored-by: spacebear <git@spacebear.dev>

Author: benalleng

payjoin-ffi/dart/pubspec.yaml

@@ -9,3 +9,4 @@ dependencies:
   ffi: ^2.1.4
 dev_dependencies:
   test: ^1.26.2
+  http: ^1.4.0