Add GetOpenAIResponseClient APIs

[eerhardt]

Similar to

aspire/src/Components/Aspire.OpenAI/AspireOpenAIClientBuilderChatClientExtensions.cs

Line 23 in 38aa880

public static ChatClientBuilder AddChatClient(

and

aspire/src/Components/Aspire.OpenAI/AspireOpenAIClientBuilderEmbeddingGeneratorExtensions.cs

Line 23 in 38aa880

public static EmbeddingGeneratorBuilder<string, Embedding<float>> AddEmbeddingGenerator(

We should have an easy API for adding GetOpenAIResponseClient to an app.

See dotnet/extensions#7407