Move ABI protobuf code to oak_abi crate

[daviddrysdale]

The ABI is currently focused on the Wasm boundary, and the host function definitions on that boundary – call this "Level 0".

Those host functions typically exchange opaque blobs of data, but in many places these opaque blobs of data are required to have a particular structure – specifically, as protobuf-encoded versions of particular messages:

• The logging pseudo-Node expects LogMessages
• The gRPC client and server pseudo-Nodes exchange GrpcRequest and GrpcResponse messages.
• The storage pseudo-Node expects GrpcRequest messages that hold StorageService requests within them.

As such, these protocol buffer definitions also form part of the Oak ABI – call this "Level 1" – and so the generated Rust code for them should live in the oak_abi crate.

Blocked on #668, because the generated code for these messages is currently rust_protobuf-generated, but oak_abi has prost-generated code for other protobuf messages

[daviddrysdale]

Not planning to move the StorageService stuff across to oak_abi, because that has a gRPC service definition as well as message definitions. The generated code for gRPC services relies on the oak SDK crate, and so can't be included in the (lower-level) oak_abi crate.