libsnapshot: Deprecate the IByteSink API.

This introduces a replacement for the IByteSink API, which was never
really used in any advantageous way, and is not expected to be useful.
The new API attempts to fill an entire buffer rather than request
individual slices of a buffer.

This patch simply introduces the API and refactors tests. Subsequent
patches will replace IByteSink callers and remove the API.

Bug: 278637212
Test: cow_api_test
Change-Id: Ib740de5e65fee8d61f603b106752338cc8e95967