Use SDK test suite and harness for C# SDK

[lisandroct]

Description of Changes

Adds tests for the C# SDK using the test suite and harness following closely what we do for Rust and Unreal.

Implementing this, I found (and fixed) three problems in the C# SDK:

• Enums could not be used as Primary Keys or Index
• Unsubscribing from a pending subscription was not supported
• Zero-width rows were not supported

API and ABI breaking changes

No breaking changes. It just adds tests and fixes three bugs.

Expected complexity level and risk

1. Very low risk. It's mostly tests (that should be passing) and only 3 source files that should be reviewed more closely.

Testing

• The new tests all pass