C#/Unity SDK - Added a multiplicity test example

[bfops]

Description of Changes

Migrating clockworklabs/com.clockworklabs.spacetimedbsdk#254 since we are merging that repo into this one.

Creates a small sample program with a Rust server and C# client that tests various subscriptions and outputs the results to the CLI.

This is a new example project that is used to test subscription multiplicity.
It currently tests for 2 scenarios:

1. A single subscription handle, using an array of subscription query with overlapping content against a single table.
   Data is modified on the server that results in both additions and removal of items from the scope of subscribed data, and the results are validated.

2. 2 subscription handles, each with a single subscription query against a single table, with overlapping content.
   Data is modified on the server that results in both additions and removal of items from the scope of subscribed data, and the results are validated. One of the subscription handles is unsubscribed to, and the results are validated.

Building the Rust server and then running the C# client from CLI will output the tests and results to the terminal.

API and ABI breaking changes

None

Expected complexity level and risk

Testing

This example project is a test itself.

• Ran the C# client and read output results, confirming expected behavior.