gRPC: implement comprehensive getters for gRPC high-level client APIs

[bingyanglin]

• Implement getter methods comprehensively for all gRPC proto types in iota-grpc-types, providing ergonomic access to proto fields.
• For every field defined in the proto messages in high-level APIs, provide a corresponding getter method.
  • High-level APIs should hide proto implementation details and return SDK types or primitives.
• Epecify the types not yet available in iota-sdk-types, considering using todo!() but it will panic.
• For the naming: for those field naming that prost already used (e.g., sequence_number() method for checkpoint), we need to use a different name to avoid the conflict.
• For the response types of these get methods we should use Result instead of Option.