refactor: add decode hook function for anypb in structmap (#434)

- Eliminate the need for special handling of asset proto by introducing
  the decode hook function for anypb type.
- Instantiate concrete type for the anypb field using the @type field in
  given map. This makes it possible into an empty asset proto.
- Verify data type and presense of data field in given map while
  decoding into an asset proto.
- Remove unused type structmap.AssetWrapper. Port the tests for it into
  structmap_test.go.

Author: sudo-suhas