feature(gRPC): add client functions

[muXxer]

We need to add client convenience functions. The clients shouldn't need to bother with the gRPC proto types.
For example in "get_transactions", the client should only pass the iota-sdk-types::Digest, and the read mask. Everything else should be hidden in the client.

We should also add functions like "get_transaction", that is a wrapper around "get_transactions" but only with a single digest passed.

All the functions should return the iota-sdk-types.

While implementing, we should check if all BCS serialized fields in the gRPC protocol have an enumeration with a version wrapped around the actual type.