Use plugin to mark classes as experimental. (Azure#43828)

* Use plugin to mark classes as experimental.

* Fix path

Author: nick863

specification/ai-foundry/data-plane/Foundry/src/sdk-csharp-azure-ai-extensions-openai/tspconfig.yaml

@@ -7,6 +7,8 @@ options:
     emitter-output-dir: "{output-dir}/{service-dir}/{package-name}"
     package-name: Azure.AI.Extensions.OpenAI
     api-version: "v1"
+    plugins:
+      - "../codegen"
   "@azure-tools/typespec-client-generator-cli":
     additionalDirectories:
       - specification/ai-foundry/data-plane/Foundry/src/agents

specification/ai-foundry/data-plane/Foundry/src/sdk-csharp-azure-ai-projects-agents/tspconfig.yaml

@@ -7,6 +7,8 @@ options:
     emitter-output-dir: "{output-dir}/{service-dir}/{package-name}"
     package-name: Azure.AI.Projects.Agents
     api-version: "v1"
+    plugins:
+      - "../codegen"
   "@azure-tools/typespec-client-generator-cli":
     additionalDirectories:
       - specification/ai-foundry/data-plane/Foundry/src/agents

specification/ai-foundry/data-plane/Foundry/src/sdk-csharp-azure-ai-projects/tspconfig.yaml

@@ -6,6 +6,8 @@ options:
     emitter-output-dir: "{output-dir}/{service-dir}/{package-name}"
     package-name: Azure.AI.Projects
     api-version: "v1"
+    plugins:
+      - "../codegen"
 imports:
   - "@azure-tools/typespec-azure-core"
   - "@azure-tools/typespec-azure-core/experimental"