mcp/server: add initial capability server option (#182)

This CL adds an option for servers to advertise support for features
that have not been added yet. This includes Prompts, Tools, and
Resources as these could be dynamically added after initialization.

Fixes: #135.

mcp/server_test.go

@@ -333,6 +333,22 @@ func TestServerCapabilities(t *testing.T) {
 				Tools:       &toolCapabilities{ListChanged: true},
 			},
 		},
+		{
+			name:            "With initial capabilities",
+			configureServer: func(s *Server) {},
+			serverOpts: ServerOptions{
+				HasPrompts:   true,
+				HasResources: true,
+				HasTools:     true,
+			},
+			wantCapabilities: &serverCapabilities{
+				Completions: &completionCapabilities{},
+				Logging:     &loggingCapabilities{},
+				Prompts:     &promptCapabilities{ListChanged: true},
+				Resources:   &resourceCapabilities{ListChanged: true},
+				Tools:       &toolCapabilities{ListChanged: true},
+			},
+		},
 	}
 
 	for _, tc := range testCases {