[Rule] breaking changes, when x-ms-discriminator-value changes

Case: https://github.com/Azure/azure-rest-api-specs/pull/20376#discussion_r959039927

Reason:
`x-ms-discriminator-value` is used in client to choose the correct type/class to initialize.

Changing its value will break SDK in runtime (there will be no API change, but SDK will fail to initialize the type in runtime), or client code (if customer uses REST API directly, and use its value to match the type).

The issue names no file, test, or entry point. Start by locating how existing breaking-change rules are represented in this C# command-line tool, then trace comparison handling for x-ms-discriminator-value. Done means a changed value is reported as breaking while unchanged values are not.

索引モデルが issue の本文から書いたものです。