Blazor Gateway: read reverse-proxy config from the app project's config files

[Copilot]

The dev-time Blazor Gateway runs with its content root set to the package's tools folder, so it never reads the WASM app project's appsettings.json / appsettings.Development.json. YARP ReverseProxy config could only be supplied via flattened ReverseProxy__... environment variables or command-line args — verbose, error-prone, and undiscoverable. This lets the gateway pick up the standard ReverseProxy section straight from the app project's config files.

Description

• BlazorGateway.BuildWebHost: new AddAppProjectConfiguration step reads an AppSettingsDirectory key and registers appsettings.json + appsettings.{Environment}.json from that directory via a PhysicalFileProvider (needed since the gateway's content root differs from the app project).
• Source precedence: the file sources are inserted just ahead of the environment-variable and command-line sources, preserving command-line > env vars > appsettings.{Environment}.json > appsettings.json. The existing env-var/launchSettings.json approach keeps working.
• Microsoft.AspNetCore.Components.Gateway.targets: run arguments now pass --AppSettingsDirectory "$(MSBuildProjectDirectory)" so the gateway knows where the app project lives (VS F5, dotnet run, etc.).
• Tests: added coverage for config-file-based ReverseProxy forwarding and for appsettings.{Environment}.json overriding the base file.

Users can now configure the proxy with the standard, IntelliSense-friendly section:

{
  "ReverseProxy": {
    "Routes": {
      "weather": { "ClusterId": "backend", "Match": { "Path": "/api/{**catch-all}" } }
    },
    "Clusters": {
      "backend": { "Destinations": { "backend1": { "Address": "http://localhost:5100/" } } }
    }
  }
}