Add option for PHP to emit default values for JSON.

[SpencerMalone]

This is a reopening of #6035, where it was requested that someone contribute a PR. I chose the option name EMIT_DEFAULTS to match the ruby client and existing upd option. I tried to be relatively comprehensive with the tests, but let me know if there's anything stylistically or logically you want changed.

Locally the tests run happily:

php % git rev-parse HEAD 

7cb3b2d9f60bdffce69c01143ca15c46d2eaa900
php % php -dextension=ext/google/protobuf/modules/protobuf.so -d error_reporting="E_ALL & ~E_DEPRECATED" vendor/bin/phpunit --bootstrap tests/force_c_ext.php tests/EncodeDecodeTest.php
PHPUnit 8.5.26 #StandWithUkraine

...............................................................  63 / 147 ( 42%)
............................................................... 126 / 147 ( 85%)
.....................                                           147 / 147 (100%)

Time: 49 ms, Memory: 6.00 MB

OK (147 tests, 1150 assertions)

[zhangskz]

I think you handled this right already from initial review, but just an FYI that the language in the guidance for option to emit default values has since been updated the original issue was opened to be more precise about explicit vs implicit presence, outside of proto3.

https://protobuf.dev/programming-guides/json/#json-options

Always emit fields without presence: Fields that don’t support presence and that have their default value are omitted by default in JSON output (for example, an implicit presence integer with a 0 value, implicit presence string fields that are empty strings, and empty repeated and map fields). An implementation may provide an option to override this behavior and output fields with their default values.

[zhangskz]

This roughly lgtm, but @mkruskal-google to review for PHP.

[SpencerMalone]

Thanks y'all!

[SpencerMalone]

What is the process for getting this cut into a release? Do I need to do anything further?

[zhangskz]

Looks like this missed the branch cut for our 33.x release, but should get picked up for our v34.0 release in Q1 which we'll be starting RC's for soon.

[SpencerMalone]

Thank you!