Support packages/custom Sdks to provide their own XSD

### Summary

Today, csproj IntelliSense is only driven by XSDs provided by MSBuild. I think it would be nice to have some sort of extensibility and allow NuGet packages and/or custom MSBuild Sdks to provide their own XSD.

### Background and Motivation

Many NuGet packages (regular ones and Sdks) offer some MSBuild properties intended for use by consumers. Today, first-party authors have to make the change in this repo, and third-party authors have no way to do it (assuming they don't go too crazy and grab the XSD from VS installation and attempt to modify it).

### Proposed Feature

Not sure if the exact shape or technical limitations here, but having something like `Sdk.xsd` (like Sdk.props and Sdk.targets that are special cased for custom Sdks) would be nice. For regular NuGet packages, it could be `build\.xsd`.

Any other design that solves the problem is also good.

### Alternative Designs

_No response_