feat(backend): add support for arbitrary provider requests with AnyRequest #114

Job	Run time
cfg-check	50s
clippy	39s
docs	49s
doctest	58s
feature-checks	40s
fmt	11s
no-std	21s
test stable --no-default-features	1m 0s
test stable	1m 2s
test stable --all-features	1m 5s
test nightly --no-default-features	1m 20s
test nightly	1m 14s
test nightly --all-features	1m 14s
test 1.79 --no-default-features	55s
test 1.79	55s
	13m 13s

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