.Net: Add sample to show how to create a decorator for generating embeddings.

[westey-m]

Motivation and Context

Generating embeddings directly every time can be frustrating and it's nice to hide this functionality away from the main code

#9483

Description

Adding a sample that shows how to create a set of decorator classes for Vector Stores to automatically generate embeddings on upsert and when using VectorizableTextSearch.

Contribution Checklist

• The code builds clean without any errors or warnings
• The PR follows the SK Contribution Guidelines and the pre-submission formatting script raises no violations
• All unit tests pass, and I have added new tests where possible
• I didn't break anyone 😄

[dmytrostruk]

Great example! I like the approach with GenerateTextEmbedding attribute!