The protobuf field type for annotations in the ResourceDescriptor should be Struct

[marcelamelara]

Prior to #251, the annotations field in the ResourceDescriptor was of type map[string]Struct. We updated it to map[string]Value to allow mapping of arbitrary values. However, we are now essentially re-implementing the structure of the Struct protobuf type. Using Struct has the advantage of Value handling from the protobuf library, rather than having to do so explicitly in our language bindings.

Since the Struct protobuf type corresponds to the intended map[string]Value type for annotations in the spec, we should update the ResourceDescriptor proto to define the annotations field as a Struct, and highlight that this still meets the spec.

Originally posted by @adityasaky in #227 (comment)