C#/Unity SDK - Added Unit Test that checks the BTreeIndexBase Column implements IComparable

[bfops]

Description of Changes

Migrating clockworklabs/com.clockworklabs.spacetimedbsdk#277 since we are merging that repo into this one.

Adds a test to com.clockworklabs.spacetimedbsdk that checks if the BTreeIndexBase class Column implements the IComparable interface. This is the implementation side of issue #2362.

API and ABI breaking changes

• This is an API breaking change to the SDK

Expected complexity level and risk

Testing

• Ran dotnet test, all checks pass.

[bfops]

Outstanding comment from the other PR:

This code won't catch problems that occur if the code generation changes in the future. To catch such problems, we'd need to write the same test against generated code. For example, we could add an index to the Message.Sender field in quickstart-chat -- these tests have access to the quickstart-chat generated code.

If you want to do that, it would be great; if not, I'm willing to merge this, but would appreciate a comment saying why this class is here. // Copy of generated code for a BTreeIndex; TODO: update if that code changes is messy but at least documents what is going on.