[PLA-2415] Enjin Platform Cloud v3

[JayPavlina]

This pull request introduces a major update for the Enjin Platform SDK, targeting the v3 GraphQL API and adding first-class Unity Package Manager (UPM) distribution support. The update consolidates all previous sub-packages into a single package, switches to .NET Standard 2.1, and replaces hand-written models with auto-generated GraphQL client code. Additionally, it provides new build scripts and workflows for Unity integration and documents the schema regeneration process.

Key changes:

Platform and Build System Updates

• Migrated the CI build workflow (.github/workflows/build.yml) from Windows/MSBuild/NuGet to Ubuntu/.NET 9/dotnet CLI, modernizing the pipeline and ensuring compatibility with the new SDK structure.
• Added a new GitHub Actions workflow (.github/workflows/unity.yml) to automate building, packaging, tagging, and releasing the Unity UPM package on every tagged release or manual trigger.
• Introduced the scripts/assemble-upm.sh script to assemble the UPM package from build outputs and templates, producing both a staging directory and a distributable tarball.

SDK and Package Structure

• Consolidated all previous sub-packages (Beam, FuelTanks, Marketplace) into a single Enjin.Platform.Sdk package, and removed all v2-specific and hand-written GraphQL models in favor of a fully auto-generated v3 client. [1] [2]
• Changed the target framework from .NET Standard 2.0 to .NET Standard 2.1 for improved compatibility with Unity 2021.3+, Godot 4+, and .NET 5+. [1] [2]

GraphQL Client Generation

• Documented the schema regeneration process in CONTRIBUTING.md, detailing how to use the platform-sdk-generators tool to produce the auto-generated client code under Schema/.
• Updated the CHANGELOG.md and README.md to reflect the new auto-generated GraphQL client, usage examples, and Unity installation instructions. [1] [2]

Unity Support

• Added comprehensive Unity installation instructions and IL2CPP/AOT notes to the README.md, including UPM installation via git URL or tarball.
• The UPM package now includes the precompiled DLL, XML docs, link.xml for code stripping, and all necessary Unity meta files.

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References:
[1] [2] [3] [4] [5] [6]

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[JayPavlina]

This doesn't include code signing on the Unity Package Manager package. It worked for me without it, but not sure if it is something we need.