Azure OpenAI: add support for optional 'name' field to ChatMessages

See originally reported request from .NET: https://github.com/Azure/azure-sdk-for-net/issues/36380

Per the linked OpenAI Python documentation,

> Messages can also contain an optional name field, which give the messenger a name. E.g., example-user, Alice, BlackbeardBot. Names may not contain spaces.

We should investigate/validate support for the `name` field on `ChatMessage` using Azure OpenAI and then, contingent on everything looking good, update the service definition and generated SDK surfaces to include it.