Add DI helpers for scenarios not requiring AI

[dluc]

Kernel Memory can be utilized to create applications that don't require Text Generation or Embedding Generation. However, these dependencies must still be set up. Given the absence of a method that is simultaneously fast, free, and of high quality for this setup — especially for embeddings — without resorting to API calls and authentication keys, the Kernel Memory builder lacks a default configuration and throws an exception (see issue #466 for an example).

To address this, we introduce two straightforward solutions that allow the injection of placeholder text and embedding generators:

• KernelMemoryBuilder.WithoutEmbeddingGenerator()
• KernelMemoryBuilder.WithoutTextGenerator()

Alternatively:

• appBuidler.Services.AddNoEmbeddingGenerator()
• appBuidler.Services.AddNoTextGenerator()

Should these placeholder classes be called upon to generate text or embeddings, they will intentionally throw a NotImplementedException exception. This ensures that any overlooked dependencies can be identified.