Config recursive access is inconsistent

[fuzzypixelz]

Describe the bug

If config: zenoh::Config then config.insert_json5("transport/unicast/max_links:2") succeeds while config.insert_json5("connect/retry/period_init_ms:500") fails with Str(\"unknown key\").

This is because the connect/retry field is an Option of a struct while transport/unicast is simply a struct. validated_struct_macros doesn't allow using recursive accessors unless the target is a simple struct:

https://github.com/p-avital/validated-struct-macros-rs/blob/048537b0ce74a7c961857c6de43436325477ffac/src/lib.rs#L392-L411

https://github.com/p-avital/validated-struct-macros-rs/blob/048537b0ce74a7c961857c6de43436325477ffac/src/lib.rs#L37-L44

The workaround is to write config.insert_json5("connect/retry:{period_init_ms:500}")

To reproduce

N/A

System info

• ref: 3740564