feat(api): Add `append` option to writeFile apis #690

Job	Run time
fmt	11s
clippy (empty)	2m 12s
clippy (--features compression,wry,linux-protocol-headers,isolation,custom-protocol,api-all,cli,u...	2m 29s
clippy (--features custom-protocol, custom-protocol)	2m 7s
clippy (--features api-all, api-all)	2m 26s
	9m 25s

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