Support to_dict for MapComposite

[dizcology]

Is your feature request related to a problem? Please describe.

It is not obvious how to turn certain proto-plus types to Python dict.

Describe the solution you'd like

If x is an instance of a proto-plus type, x.to_dict() is a Python dict representation of it.

Additional context

As an example:

import proto
from google.cloud import dialogflowcx_v3beta1 as df

qr = df.QueryResult(parameters={'some_key': 'some_value'})

print(type(qr.parameters))  # proto.marshal.collections.maps.MapComposite

dict(qr.parameters)  # this works, but not intuitive.

qr.parameters.to_dict()  # no such method!

proto.Message.to_dict(qr)  # this works, but not intuitive.

qr.to_dict()  # no such method!

Bonus, continuing:

print(dict(qr.parameters))  # {'some_key': 'some_value'}, good!

print(dict(qr.parameters.pb)  # {'some_key': string_value: "some_value"}, huh?