[Feature] Online Config Cache

[database64128]

Generated online config JSON can be cached until the data service reloads.

Blocked by Response Caching Middleware's lack of support for cache eviction.

• dotnet/aspnetcore#2622
• dotnet/aspnetcore#27387

An alternative is to implement our own in-memory cache.

References:

• Response caching in ASP.NET Core | Microsoft Docs
• Cache in-memory in ASP.NET Core | Microsoft Docs
• Response Caching Middleware in ASP.NET Core | Microsoft Docs
• A guide to caching in ASP.NET Core
• Custom response caching in ASP.NET Core (with cache invalidation)