Fixes mehrandvd#1

Add support for testing OpenAI plugins using their URL.

* **README.md**:
  - Add a section explaining how to test OpenAI plugins using their URL.
  - Include an example of testing a plugin using its URL.
* **src/skUnit/Asserts/SemanticKernelAssert_Chat.cs**:
  - Add a new method `CheckPluginByUrlAsync` to test chat scenarios using a plugin URL.
  - Implement the method to load the plugin from the URL and run the chat scenario.
* **src/skUnit/Asserts/SemanticKernelAssert_Function.cs**:
  - Add a new method `CheckPluginByUrlAsync` to test function scenarios using a plugin URL.
  - Implement the method to load the plugin from the URL and run the function scenario.
* **src/skUnit/Asserts/SemanticKernelAssert_Initialize.cs**:
  - Add a new constructor to initialize `SemanticKernelAssert` with a plugin URL.
  - Implement the constructor to load the plugin from the URL and initialize the kernel.

Refactored SemanticTestBase to use ScenarioAssert and IChatClient. Updated KernelChatTests to use ScenarioAssert. Removed obsolete test classes and methods, including FunctionTests and KernelTests. Deleted config.json, sktest.md, and skprompt.txt files. Updated skUnit.Tests.csproj to remove references to deleted files and scenarios. Updated ScenarioAssert_Initialize.cs and SemanticAssert.cs to use IChatClient. Removed PocomoPlugin.cs and related configurations. Updated skUnit.csproj to target .NET 8.0 and updated package references. Added new ChatClientTests class in ChatClientTests.cs. Introduced new methods in ScenarioAssert for chat scenario validation.