Add add_resource_template() to MCPServer

Description

MCPServer and ResourceManager have add_resource() method for static Resources, but there is no way to handle ResourceTemplate without using @resource decorator.

I am implementing the following workaround using the decorator for my usecase, but it is a bit odd.

server = MCPServer("...")

# Configure resources (implemented in other files)
server.resource("resource://users")(resources.list_users)
server.resource("resource://users/{id}")(resources.get_user)

References

It was implemented in #3208.