feat: modernize GitHub Actions with security hardening and centralized Rust toolchain (#470)

Security Improvements:
- Pin all GitHub Actions to specific commit hashes instead of version tags
- Update actions/checkout from v4 to commit 08eba0b27e820071cde6df949e0beb9ba4906955
- Update actions/setup-python from v5 to commit a26af69be951a213d495a4c3e4e4022e16d87065 (v5.6.0)
- Update actions/setup-java from v4 to commit dded0888837ed1f317902acf8a20df0ad188d165 (v5.0.0)
- Update actions/setup-node from v4 to commit 1e60f620b9541d16bece96c5465dc8ee9832be0b (v4.4.0)
- Update actions/setup-go from v5 to commit 41dfa10bad2bb2ae585af6ee5bb4d7d973ad74ed (v5.1.0)
- Update actions/setup-dotnet from v4 to commit 3e891b0cb619bf60e2c25674b222b8940e2c1c25 (v4.1.0)
- Update actions/upload-artifact from v4 to commit ea165f8d65b6e75b540449e92b4886f43607fa02 (v4.6.2)
- Update actions/download-artifact from v4 to commit 634f93cb2916e3fdff6788551b99b062d0335ce0 (v5.0.0)
- Update github/codeql-action from v3 to commit 01fe2e8c43536ad5e1085bad5e7cd6fbc8a30988 (v3.29.11)

Rust Toolchain Consolidation:
- Create custom composite action .github/actions/toolchains/rust/action.yml
- Standardize on Rust 1.89.0 (latest stable) with clippy and rustfmt components
- Add optional targets parameter for cross-compilation support
- Replace dtolnay/rust-toolchain@stable across 16 workflows

This creates a more secure, maintainable, and consistent CI/CD pipeline
with centralized Rust toolchain management across all workflows.

Signed-off-by: Anand Krishnamoorthi <anakrish@microsoft.com>

.github/workflows/test-csharp.yml

@@ -38,9 +38,10 @@ jobs:
           #    **/release/libregorus_ffi.dylib
     steps:
       - name: Checkout repository
-        uses: actions/checkout@v4
+        uses: actions/checkout@08eba0b27e820071cde6df949e0beb9ba4906955 # v4
         with:
           fetch-depth: 0
+      - uses: ./.github/actions/toolchains/rust
       
       - name: Fetch crates
         run: cargo fetch
@@ -59,7 +60,7 @@ jobs:
         working-directory: ./bindings/ffi
         
       - name: Upload regorus ffi shared library
-        uses: actions/upload-artifact@v4
+        uses: actions/upload-artifact@ea165f8d65b6e75b540449e92b4886f43607fa02 # v4.6.2
         with:
           name: regorus-ffi-artifacts-${{ matrix.runtime.target }}
           # Note: The full path of each artifact relative to . is preserved.
@@ -73,17 +74,17 @@ jobs:
     needs: build-ffi
     steps:
       - name: Checkout repository
-        uses: actions/checkout@v4
+        uses: actions/checkout@08eba0b27e820071cde6df949e0beb9ba4906955 # v4
         with:
           fetch-depth: 0  
-      - uses: actions/setup-dotnet@v4
+      - uses: actions/setup-dotnet@3e891b0cb619bf60e2c25674b222b8940e2c1c25 # v4.1.0
         with:
           global-json-file: ./bindings/csharp/global.json
 
       - run: echo '${{ steps.stepid.outputs.dotnet-version }}'
 
       - name: Download regorus ffi shared libraries
-        uses: actions/download-artifact@v4
+        uses: actions/download-artifact@634f93cb2916e3fdff6788551b99b062d0335ce0 # v5.0.0
         with:
           pattern: regorus-ffi-artifacts-*
           merge-multiple: true
@@ -102,7 +103,7 @@ jobs:
         working-directory: ./bindings/csharp/Regorus
 
       - name: Upload Regorus nuget
-        uses: actions/upload-artifact@v4
+        uses: actions/upload-artifact@ea165f8d65b6e75b540449e92b4886f43607fa02 # v4.6.2
         with:
           name: regorus-nuget
           path: bindings/csharp/Regorus/bin/Release/Regorus*.nupkg
@@ -126,18 +127,18 @@ jobs:
           #  target: aarch64-apple-darwin
     steps:
       - name: Checkout repository
-        uses: actions/checkout@v4
+        uses: actions/checkout@08eba0b27e820071cde6df949e0beb9ba4906955 # v4
         with:
           fetch-depth: 0
 
-      - uses: actions/setup-dotnet@v4
+      - uses: actions/setup-dotnet@3e891b0cb619bf60e2c25674b222b8940e2c1c25 # v4.1.0
         with:
           global-json-file: ./bindings/csharp/global.json
 
       - run: echo '${{ steps.stepid.outputs.dotnet-version }}'
 
       - name: Download regorus nuget
-        uses: actions/download-artifact@v4
+        uses: actions/download-artifact@634f93cb2916e3fdff6788551b99b062d0335ce0 # v5.0.0
         with:
           name: regorus-nuget
           path: ./bindings/csharp/regorus-nuget/