Enable default features in zenoh-ext/examples

[YuanYuYuan]

Describe the bug

Similar issue as #854. If we run examples under zenoh-ext instead of workspace, then the default features of zenoh are missing to include.

To reproduce

cd zenoh-ext
cargo build --example z_pub_cache --example z_query_sub
cd ..
# Terminal 1
./target/debug/examples/z_pub_cache
# Terminal 2
./target/debug/examples/z_query_sub

Symptom: The querying side never receives a message.

System info

• OS: Arch Linux x86_64
• Kernel: 6.8.2-arch2-1

[Mallets]

Similar to what done in zenoh where a different examples module has been created, also in zenoh-ext we should have a separate examples module to avoid cargo features conflict. See https://github.com/eclipse-zenoh/zenoh/pull/905.