Create extractResourceOwnerAndBucket() helper to consolidate the repeated pattern
of unmarshaling metadata and extracting bucket name from resource path. This
pattern was duplicated in handleTagResource, handleListTagsForResource, and
handleUntagResource. Update all three handlers to use the helper.
Also update remaining uses of getPrincipalFromRequest() (in handler_bucket_create,
handler_bucket_get_list_delete, handler_namespace) to use getAccountID() after
consolidating the two identical methods.
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