New endpoint customize callback to provide settings dynamically

[lminiero]

This PR adds some dynamicity to endpoints creation. While you can still provide endpoint properties statically (e.g., room to join, label to use as display, RTP forwarding recipients, etc.), you can now also provide a customize callback function, where you can provide these properties dynamically any time a new publisher contacts an endpoint. This can be useful to, e.g., send different publishers of the same endpoint to different rooms, or configure different RTP forwarders for each publisher, and things like that. The PR comes with a basic example (server-dynamic) that shows it being used to customize the label.