[C#] Add support for IEnumerable<T> for Views' return types

[rekhoff]

This is the implementation to resolve #4424

Description of Changes

• Add support for IEnumerable<T> as a valid C# View return type in codegen, so view implementations can return query results without forcing ToList(). The generator now detects IEnumerable<T>, serializes it via the list serializer.
• Snapshots have been updated with a test to confirm that IEnumerable<T> rather than checking it is unsupported (which we had added when removing support during the transition away from Query<T> usage.
• Refreshes docs/examples to reflect the new supported signature.

API and ABI breaking changes

No breaking changes. This is additive support for an existing .NET interface.

Expected complexity level and risk

2 - Low

Testing

• Rebuilt CLI locally, ran regression tests, and dotnet tests in C# module bindings.
• Tested updated docs code snippets to ensure they resolve in an IDE and publish.

[joshua-spacetime]

Do both of these options implement IEnumerable? If so, we could just assert IEnumerable or IQuery.