Add prerender-aspnetcore middleware for ASP.NET Core 8+

[Lasalot]

Summary

• IMiddleware + IHttpClientFactory pattern — idiomatic ASP.NET Core, avoids socket exhaustion
• Zero external dependencies beyond the ASP.NET Core framework
• Settings via IConfiguration / appsettings.json (Prerender:Token, Prerender:ServiceUrl)
• Same bot list and extension ignore list as other integrations (hapi, koa, django)
• xUnit tests using Microsoft.AspNetCore.TestHost with a mock HttpMessageHandler
• NuGet package metadata ready (Prerender.AspNetCore v1.0.0)

Usage

builder.Services.AddPrerender();
app.UsePrerender();

{ "Prerender": { "Token": "your-token" } }

Test plan

• dotnet test passes all 7 test cases
• Browser UA → passes through
• Bot UA → receives prerendered response
• Bot UA + static asset → passes through
• _escaped_fragment_ → prerendered
• X-Bufferbot header → prerendered
• POST + bot UA → passes through
• Network error → graceful fallback
• dotnet pack produces valid NuGet package