assets: add support for configurable bufsize for FileHandler (#649) #447

Job	Run time
clippy	14s
rustfmt	10s
coverage	9m 41s
test-stable	5m 36s
test-gotham	3m 2s
test-beta	5m 3s
test-gotham--all-features	3m 22s
test-nightly	4m 49s
test-gotham--no-default-features	30s
	32m 27s

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