HashingReader wraps an existing AsyncRead, and allows querying for the
digest of all data read "through" it.
The hash function is configurable by type parameter, and we define
B3HashingReader.
Change-Id: Ic08142077566fc08836662218f5ec8c3aff80be5
Reviewed-on: https://cl.tvl.fyi/c/depot/+/11087
Autosubmit: flokli <flokli@flokli.de>
Reviewed-by: raitobezarius <tvl@lahfa.xyz>
Tested-by: BuildkiteCI
