feat: add serde for OpTxType (#317) #1159

Job	Run time
clippy	45s
cfg-check	21s
docs	26s
doctest	46s
examples	1m 1s
feature-checks	3m 17s
feature-powerset	4m 29s
fmt	13s
no-std	46s
test stable --no-default-features	1m 1s
wasm-unknown	1m 1s
wasm-wasi	1m 0s
test stable	1m 15s
test stable --all-features	1m 26s
test nightly --no-default-features	49s
test nightly	55s
test nightly --all-features	1m 19s
test 1.81 --no-default-features	49s
test 1.81	46s
	22m 25s

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