mcp: make transports open structs

As described in #272, there is really no reason for transports to be
closed structs with constructors, since their state must be established
by a call to Connect. Making them open structs simplifies their APIs,
and means that all transports can be extended in the future: we don't
have to create empty Options structs just for the purpose of future
compatibility.

For now, the related constructors and options structs are simply
deprecated (with go:fix directives where possible). A future CL will
remove them prior to the v1.0.0 release.

For #272

mcp/sse_example_test.go

@@ -35,7 +35,7 @@ func ExampleSSEHandler() {
 	defer httpServer.Close()
 
 	ctx := context.Background()
-	transport := mcp.NewSSEClientTransport(httpServer.URL, nil)
+	transport := &mcp.SSEClientTransport{Endpoint: httpServer.URL}
 	client := mcp.NewClient(&mcp.Implementation{Name: "test", Version: "v1.0.0"}, nil)
 	cs, err := client.Connect(ctx, transport, nil)
 	if err != nil {