pw_kvs: Return StatusWithSize from Item::Get
pw_kvs: Return StatusWithSize from Item::Get

This change updates the item class to return a StatusWithSize when
reading a value so that the caller knows how large the value is without
requiring an additional flash read.

Change-Id: I37c8c2b5f39ecd3d2f4c37c3a7af136489ff332a

google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed