Include `PromptType` and `Prompt` attributes

[aaronpowell]

Is your feature request related to a problem? Please describe.
The API for McpToolType and McpTool (combined with the WithTools method) makes it really easy to define tools. It'd be great to have a comparable API for Prompts.

Describe the solution you'd like
I'd like to be able to have code like this:

[PromptType]
public static class SamplePrompt {
  [Prompt, Description("This is a sample prompt")]
  public static string Sample() => "Some sample prompt";
}

// in program.cs
builder.Services
    .AddMcpServer()
    .WithStdioServerTransport()
    .WithTools()
    .WithPrompts();

Describe alternatives you've considered
Currently, the only solution would be to implement this via WithListPromptsHandler and WithGetPromptHandler.