Storage: support signed URL on GAE and GCE

[neozwu]

Today signed url requires a service account with a signer. GAE and GCE use their built-in service account, which cannot sign blob. Our current impl only retrieves signer from service account. Therefore, it is not working on GAE and GCE. GAE has appIdentity.signForApp API that can be used to sign blob. It seems there's ongoing work to support GCE to sign blob. We should make GAE ( and GCE once it's ready) blob signing available in client lib as well.

[garrettjonesgoogle]

@neozwu there are two tasks in here: 1) GAE, 2) GCE. Could you split this apart?

[neozwu]

GAE & GCE now both use metadata server. Close this issue since supporting signing URL is tracked in #2629