Add C# language bindings for payjoin-ffi (#1318)

Author: spacebear21

.github/workflows/csharp.yml

@@ -0,0 +1,44 @@
+name: Build and Test CSharp
+on:
+  pull_request:
+    paths:
+      - payjoin-ffi/**
+env:
+  RUSTUP_TOOLCHAIN: 1.85
+
+jobs:
+  build-csharp-and-test:
+    name: "Build and test csharp"
+    runs-on: ${{ matrix.os }}
+    defaults:
+      run:
+        working-directory: payjoin-ffi/csharp
+    strategy:
+      matrix:
+        os: [ubuntu-latest, macos-latest, windows-latest]
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+
+      - name: Install Rust 1.85.0
+        uses: dtolnay/rust-toolchain@1.85.0
+
+      - name: Use cache
+        uses: Swatinem/rust-cache@v2
+
+      - name: Install .NET 8 SDK
+        uses: actions/setup-dotnet@v4
+        with:
+          dotnet-version: "8.0.x"
+
+      - name: Generate bindings and binaries (unix)
+        if: matrix.os != 'windows-latest'
+        run: bash ./scripts/generate_bindings.sh
+
+      - name: Generate bindings and binaries (windows)
+        if: matrix.os == 'windows-latest'
+        shell: pwsh
+        run: ./scripts/generate_bindings.ps1
+
+      - name: Run tests
+        run: dotnet test --logger "console;verbosity=minimal"