featue leak fixed, test added

[milyin]

there was a leak of zenoh's default features through zenoh-ext. Fixed it, added test to zenohd to control this in future

[Mallets]

This PR looks good to me, however I believe it may break our plugins and bindings in cascade without additional PRs.
E.g., zenoh-c needs to explicitly add default-features = false when including zenoh and add default = ["zenoh/default"] for the crate's default features.

[milyin]

This PR looks good to me, however I believe it may break our plugins and bindings in cascade without additional PRs. E.g., zenoh-c needs to explicitly add default-features = false when including zenoh and add default = ["zenoh/default"] for the crate's default features.

There is no break for dependent crates/projects. zenoh-c is not required to add "default-features = false", it just can add this parameter now and be sure that no unnecessary features leak out through another crate.
I.e. it's still possible to include zenoh with or without default features, as before. The only difference that now including zenoh with default-features = false really works