Add support to display the default values.

[ghevge]

Describe the bug

Evans doesn't offer the possibility to display the default values (something similar to -emit-defaults in grpccurl).

To reproduce

View the response fields which were set to their default value . Those fields will not be visible at all in evans console, as the grpc doesn't return them , per grpc specs: https://developers.google.com/protocol-buffers/docs/proto3#json

Expected behavior

default fields should me made available in the evans console, on demand

Screenshots

N/A

Environment

All

Additional context

Issue opened based on this discussion: #403 (comment)