Add possibility to show only fields with non-default values

[n0v1]

It looks like grpcui always displays all fields defined in the proto in the response tab. Would it be possible to make this configurable? We would like to show only fields with non-default values (e.g. values != 0 for int fields) similar to what grpcurl does by default (when not setting -emit-defaults). Is this feasible?

With some guidance on where to look I can try to provide a pull request.