feat: Implement brand-aware release creation for Agravity

- Added support for multiple brands in release scripts, allowing for branded artifacts.
- Introduced brand configuration management with JSON files for each brand.
- Created a new `brand_config.py` script to handle brand-specific logic and asset resolution.
- Updated `create_release.ps1` and `create_release.sh` scripts to utilize brand configurations and generate release manifests.
- Added unit tests for brand configuration loading and release manifest generation.
- Introduced `agravity` brand with its specific configuration in `agravity.json`.

build/brands/agravity.json

@@ -0,0 +1,18 @@
+{
+  "brand_id": "agravity",
+  "display_name": "Agravity Bridge",
+  "asset_prefix": "AgravityBridge",
+  "exe_name": "AgravityBridge",
+  "manufacturer": "agravity",
+  "install_dir_name": "Agravity Bridge",
+  "shortcut_description": "Agravity drag-and-drop bridge",
+  "bundle_identifier": "io.agravity.bridge",
+  "config_dir_name": "agravity_bridge",
+  "msi_upgrade_code": "4a7c80da-6170-4d88-8efc-3f30636f6392",
+  "update_channel": "stable",
+  "icon_ico": "resources/icons/app.ico",
+  "icon_icns": "resources/icons/app.icns",
+  "dialog_bmp": "resources/icons/background.bmp",
+  "banner_bmp": "resources/icons/banner.bmp",
+  "license_rtf": "resources/license.rtf"
+}