Config defaults

[OlivierHecart]

The first commit gathers config defaults as modules and constants in zenoh_config::defaults.

The second commit may be more controversial. It redefines listen.endpoints and connect.endpoints as optional ModeDependentValues. This is more accurate and allows users to explicitly set an empty list of listen endpoints and instruct Zenoh not to open any listening port. But while remaining backward compatible concerning the config file format, it slightly changes the way to access an update the conf programmatically. Thus strictly speaking it may be considered an API change.
Note that connect.endpoints did not need to be retyped to a ModeDependentValue, but it makes things more homogeneous to have both listen.endpoints and connect.endpoints having the same type and be accessed the same way.

[fuzzypixelz]

@OlivierHecart Please change your pull request's base branch to main (new default branch). And rebase your branch against main as it is missing a status check necessary to merge this pull request but which is only available on main.

[imstevenpmwork]

@OlivierHecart This draft PR has been stalled for more than two years. Consider closing it if it is not longer relevant (you can always open it again whenever it is needed without losing any progress)