Add URI Protocol Handler Add-on support

[kallisti5]

Haiku's network stack should include support for add-on URI handlers.

The intended location is here:

• ​https://git.haiku-os.org/haiku/tree/src/kits/network/libnetapi/UrlProtocolRoster.cpp

This would be a requirement for writing something like #16477 out of tree.

Leaving "critical" URL handlers like HTTP, HTTPS, file, data, etc built in is likely desirable since we don't want anyone "hijacking" http handling.