Make the # of threads used for tokio configurable as was done in the vsomeip transport

For reference, [here's](https://github.com/eclipse-uprotocol/up-transport-vsomeip-rust/blob/d659287126120d4feb75cbcc35a5334def421f50/up-transport-vsomeip/src/lib.rs#L123-L126) how it's done in the vsomeip transport.