feat(Storage): support object retention lock

[vishwarajanand]

This adds support for object retention lock

Fixes #6741
PRD: go/gcs-object-retention-client-request

Tests

System tests pass in my local which were run on an allowlisted project:

➜  Storage git:(main) ✗ vendor/bin/phpunit  -c phpunit-system.xml.dist --filter="ManageObjectsTest::testObjectRetentionLock"
PHPUnit 9.6.13 by Sebastian Bergmann and contributors.

.                                                                   1 / 1 (100%)

Time: 00:04.678, Memory: 14.00 MB

OK (1 test, 3 assertions)
➜  Storage git:(main) ✗

[yash30201]

Also, we should test to check the case where we upload an object in locked retention mode and then try to change the retention:

1. Decrease / Remove it => It should throw an exception from backend.
2. Increase it => This should be possible.

Ref: User Guide (go/object-lock-user-guide)

[vishwarajanand]

Also, we should test to check the case where we upload an object in locked retention mode and then try to change the retention:

1. Decrease / Remove it => It should throw an exception from backend.
2. Increase it => This should be possible.

Ref: User Guide (go/object-lock-user-guide)

I added testObjectRetentionLockedMode, but it needs more changes in tests side which I will do later. Filed a bug for that: b/315469910

[yash30201]

Lgtm