Adding custom mcp server is too manual

[davidfowl]

Explicit adding McpServerEndpointAnnotation feels a little raw. Let's add an extension method for this.

Current pattern

var api = builder.AddCSharpApp("api", "./api")
    .WithAnnotation(McpServerEndpointAnnotation.FromEndpoint("https"));

This is only discoverable if you know about McpServerEndpointAnnotation. It should be something like:

var api = builder.AddCSharpApp("api", "./api")
    .WithMcpServer("/mcp");  // or .WithMcpServerEndpoint("https")

The extension method could handle the annotation creation and optionally take the MCP endpoint path for validation/documentation purposes.