feat(storage): add Cloud Storage bucket IP filtering samples

[salilg-eng]

This PR add PHP code samples demonstrating how to manage IP filtering for Cloud Storage buckets.

What's included:

• Snippets for getting, enabling, disabling, listing, and deleting bucket IP filtering rules.
• A full lifecycle integration test (IpFilterTest.php) to cover the new snippets.

Note on enable_ip_filtering.php:
I set the mode to Disabled by default in the snippet (with an accompanying comment). If we actually set it to Enabled during the automated tests, we risk locking the test runner out of the bucket and causing permission failures during the cleanup phase. Users can easily flip it to Enabled when adapting the code for their own use.

[snippet-bot]

Here is the summary of changes.

You are about to add 5 region tags.

• storage/src/delete_ip_filtering_rules.php:26, tag storage_delete_ip_filtering_rules
• storage/src/disable_ip_filtering.php:26, tag storage_disable_ip_filtering
• storage/src/enable_ip_filtering.php:26, tag storage_enable_ip_filtering
• storage/src/get_ip_filtering.php:26, tag storage_get_ip_filtering
• storage/src/list_buckets_ip_filtering.php:26, tag storage_list_buckets_ip_filtering

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[gemini-code-assist]

Code Review

This pull request introduces new PHP code samples for managing Cloud Storage bucket IP filtering rules, including enabling, disabling, retrieving, deleting, and listing IP filtering configurations, along with a lifecycle integration test. The feedback suggests improving the usability of the enable_ip_filtering sample by making the filter mode an optional parameter (defaulting to 'Enabled') and passing 'Disabled' explicitly in tests to prevent runner lockout. Additionally, it is recommended to specify the 'full' projection when listing buckets to ensure the ipFilter metadata is reliably returned.