Support "Emit fields with default values"

[rty813]

JSON options

A proto3 JSON implementation may provide the following options:
Emit fields with default values: Fields with default values are omitted by default in proto3 JSON output. An implementation
may provide an option to override this behavior and output fields with their default values.

Given, for example, the following proto definitions:

message X{
  int n = 1;
}

my code:

x = X();
print(x.toProto3Json());

Current output: {}
What I want: {n: 0}

[osa1]

Duplicate of #585 (PR #592).