refactor: make RpcCall an IntoFuture #5707

Job	Run time
doctest	46s
wasm-wasi	32s
clippy	28s
check no_std	27s
docs	24s
fmt	13s
wasm-unknown	49s
test ubuntu-latest stable --no-default-features	45s
test ubuntu-latest stable	35s
feature-checks	26s
test ubuntu-latest stable --all-features	1m 0s
test ubuntu-latest nightly --no-default-features	36s
test ubuntu-latest nightly	48s
test ubuntu-latest nightly --all-features	58s
test ubuntu-latest 1.81 --no-default-features	45s
test ubuntu-latest 1.81	46s
test windows-latest stable --no-default-features	2m 41s
test windows-latest stable	3m 57s
test windows-latest stable --all-features	4m 8s
test windows-latest nightly --no-default-features	2m 9s
deny / cargo deny check	21s
test windows-latest nightly	2m 57s
test windows-latest nightly --all-features	4m 0s
test windows-latest 1.81 --no-default-features	1m 51s
test windows-latest 1.81	1m 50s
ci success	4s
	34m 16s

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