Add cargo ci dlls command for building C# DLLs and NuGet packages

[rekhoff]

Description of Changes

• Added a new cargo ci dlls subcommand to build/pack the in-repo C# NuGet packages and the C# SDK.
• cargo ci dlls restores sdks/csharp/SpacetimeDB.ClientSDK.csproj using the freshly built local package outputs as to populate sdks/csharp/packages/**.
• Added a Unity .meta skeleton under sdks/csharp/unity-meta-skeleton~/** and overlays those .meta files onto the latest restored versioned package directory to keep Unity GUIDs stable and import settings consistent.
• Unity-specific import fixes are captured in the skeleton overlay (notably: preventing Unity from importing incompatible TFMs like net8.0, and marking analyzer DLLs with the RoslynAnalyzer label so Unity can recognize them).

How to use (local)

# Build/pack + restore local packages into sdks/csharp/packages/**
cargo ci dlls

API and ABI breaking changes

N/A

Expected complexity level and risk

2 - Local developer tooling + file overlay into restore output; no runtime/SDK API behavior changes.

Testing

• cargo check -p ci
• Ran cargo ci dlls and verified the output under sdks/csharp/packages/** and the various NuGet package locations.
• Tested a Unity project importing the SpacetimeDB SDK after generating output and confirmed no errors.