docs: Adding Samples for Add/Remove Bucket Default Owner

[sydney-munro]

Internal issues:

• b/219968675 storage_add_bucket_default_owner (JAVA)
• b/219968340 storage_remove_bucket_default_owner (JAVA)

[snippet-bot]

Here is the summary of changes.

You are about to add 2 region tags.

• samples/snippets/src/main/java/com/example/storage/bucket/AddBucketDefaultOwner.java:19, tag storage_add_bucket_default_owner
• samples/snippets/src/main/java/com/example/storage/bucket/RemoveBucketDefaultOwner.java:19, tag storage_remove_bucket_default_owner

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[BenWhitehead]

LGTM, if either @JesseLovelace or @frankyn could also give this a quick review that'd be great.