Validate attributes at first opportunity

[mandarjog]

Mixer does not validate attributes sent by Proxy or attributes generated by an adapter with the attribute vocabulary.

This means that

1. Validation code is strewn all over the code base.
2. We assume validity and incur a panic if an incorrect type is sent.

This was seen in https://github.com/istio/mixer/pull/1342

[geeknoid]

I think we need to augment the attr package with knowledge of attribute manifests so that it can provide a typed experience around attributes.