Marking that the converter function in the derived actor is cross-thr… #687

Job	Run time
rustfmt	12s
Clippy	1m 4s
Run the default tests	3m 8s
Test ractor without async-trait	3m 15s
Test ractor without span propogation	3m 17s
benches	3m 27s
docs	1m 19s
Test ractor with the `cluster` feature	3m 5s
Test ractor with the `blanket_serde` feature	3m 6s
Test ractor_cluster with native async traits	2m 45s
Test ractor_cluster with async_trait	2m 40s
Test ractor with the monitor API	3m 8s
	30m 26s

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