feat: Add autopopulated proto3 optional field to echo

[parthea]

To test the The field supports explicit presence, and has not been set by the user requirement of AIP-4235

The field must be automatically populated if and only if one of the following conditions holds:

The field supports explicit presence, and has not been set by the user
The field doesn't support explicit presence, and its value is the empty string (i.e. the default value)