Make the default Session configurable

[JEnoch]

The default Session uses the default Config. If this doesn't suit for the user, he's forced to create another Session with a config file which is cumbersome.

It would be nice to make the default Session configurable, especially with nuze accepting similar arguments than the Zenoh examples: https://github.com/eclipse-zenoh/zenoh/blob/main/examples/src/lib.rs

[fuzzypixelz]

The way to do this today is:

$ nuze --no-default-session -e 'zenoh open {id:a123,mode:router}'

-e (--execute) will drop you into the REPL and --no-default-session means only one session will be opened. You can write your config inline which covers all options.