Dictionary representation improvement?

Is there a better way to represent this dictionary into a `Struct()`? (Currently using 2.0.0b3)

```python
capability_dict = {"@context": "https://w3id.org/security/v2",
"target": "urn:trinsic:wallets:noop",
"proof": {
"created": datetime.datetime.now()
}
}
```
The following does work, but seems pretty kludgy:
```python
proof = Struct()
proof.fields["created"] = Value(string_value=str(datetime.datetime.now()))
capability = Struct()
capability.fields["@context"] = Value(string_value="https://w3id.org/security/v2")
capability.fields["target"] = Value(string_value="urn:trinsic:wallets:noop")
capability.fields["proof"] = Value(struct_value=proof)
```

* It seems to me that we could/should at least have a `Struct.from_dictionary(obj: dict)` method. I'd be happy to write this if it doesn't exist.
* `Struct.from_dict()` does NOT work for this, tested.

Example code of what I'm thinking:
```python
def value_to_proto_value(obj: Any) -> Value:
value = Value()
if isinstance(obj, str) or isinstance(obj, datetime.datetime):
value = Value(string_value=str(obj))
elif isinstance(obj, int) or isinstance(obj, float):
value = Value(number_value=float(obj))
elif isinstance(obj, bool):
value = Value(bool_value=obj)
elif isinstance(obj, dict):
value = Value(struct_value=dictionary_to_struct(obj))
elif isinstance(obj, list):
value = Value(list_value=list_to_proto_list(obj))

return value

def list_to_proto_list(obj: List) -> ListValue:
return ListValue(values=[value_to_proto_value(item) for item in obj])

def dictionary_to_struct(obj: Dict[str, Any]) -> Struct:
return Struct(fields=dict([(k, value_to_proto_value(v)) for k, v in obj.items()]))
```

Start by locating the Struct, Value, and ListValue definitions and inspecting why Struct.from_dict() does not handle the shown nested dictionary. Confirm the intended conversion behavior for nested dictionaries, lists, strings, numbers, booleans, and datetime values. Done means an agreed ergonomic API and coverage for the example representation.

Scritto dal modello di indicizzazione a partire dal testo della issue.