CLU SDK missing polymorphic input documentation for `ConversationItemBase`

Related to Azure/azure-sdk-for-net#29443, I noticed that there are models missing the `discriminator` and [`x-ms-discriminator-value`](https://github.com/Azure/autorest/tree/main/docs/extensions#x-ms-discriminator-value) across 2022-05-01 APIs, including:

* [analyzeconversation.json](https://raw.githubusercontent.com/Azure/azure-rest-api-specs/725f4ca360426a32d20e81eb945065e62c285d6a/specification/cognitiveservices/data-plane/Language/stable/2022-05-01/analyzeconversations.json)
* `ConversationItemBase`
* `TextConversationItem`
* [analyzeconversation-authoring.json](https://raw.githubusercontent.com/Azure/azure-rest-api-specs/725f4ca360426a32d20e81eb945065e62c285d6a/specification/cognitiveservices/data-plane/Language/stable/2022-05-01/analyzeconversations-authoring.json)
* `ConversationalAnalysisAuthoringJobState`
* `ConversationalAnalysisAuthoringDeploymentJobState`
* `ConversationalAnalysisAuthoringJobState`
* `ConversationalAnalysisAuthoringImportProjectJobState`
* `ConversationalAnalysisAuthoringProjectDeletionJobState`
* `ConversationalAnalysisAuthoringTrainingJobState`

These should probably be fixed in the 2022-07-01-preview version of swagger, but we can fix them for now in code. That just won't scale well if/when changes are made to the swagger.

/cc @kristapratico @mshaban-msft