Add proper span handling to factories through extension trait #643

Job	Run time
Clippy	1m 7s
benches	3m 24s
docs	1m 12s
rustfmt	10s
Run the default tests	3m 20s
Test ractor without span propogation	3m 13s
Test ractor with the `cluster` feature	3m 25s
Test ractor with the `blanket_serde` feature	3m 31s
Test ractor_cluster	3m 10s
	22m 32s

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