Make SK text and embedding generator wrappers public

[dluc]

Currently it's possible to inject Semantic Kernel classes into KM only using

• IKernelMemoryBuilder.WithSemanticKernelTextGenerationService
• IKernelMemoryBuilder.WithSemanticKernelTextEmbeddingGenerationService
• IServiceCollection.AddSingleton(instance)
• IServiceCollection.AddSingleton(instance)

However, it's not possible to create an instance of Semantic Kernel classes directly without writing a custom wrapper class first. Since the project already includes such wrapper, as internal classes, this PR changes them to public classes, to reduce the amount of code to write.

Classes:

• SemanticKernelTextGenerator
• SemanticKernelTextEmbeddingGenerator

[dluc]

See also #801