datastore: try to decode GAE-encoded keys

If a key can't be decoded, try to decode it with the encoding scheme from
google.golang.org/appengine/datastore.

Along with golang/appengine#192, this allows for
cross-compatibility for encoded keys across the two libraries.

Co-authored-by: Luke Mccoy <[email protected]>
Co-authored-by: Chris Broadfoot <[email protected]>

Change-Id: I7e091b37b184a12c30998c3c22baffb964018a61
Reviewed-on: https://code-review.googlesource.com/c/gocloud/+/41370
Reviewed-by: kokoro <[email protected]>
Reviewed-by: Tyler Bui-Palsulich <[email protected]>
Reviewed-by: Jean de Klerk <[email protected]>
Reviewed-by: Steven Buss <[email protected]>

Author: goog-lukemc