feat(Storage): update StorageObject::exists to use HEAD request instead of GET

[salilg-eng]

Fixes / Addresses

Addresses b/437200793

Behavior Change & Motivation

Previously, StorageObject::exists() invoked getObject() (making an HTTP GET request) to verify if an object existed. Following a server-side enhancement to Google Cloud Storage Audit Logs, failed GET requests against non-existent objects started emitting severity ERROR log messages rather than INFO.

Because checking whether an object exists is intended for workflows where an object may not be present, it should not generate false-alarm error severity records in Cloud Audit Logs or trigger unnecessary payload data downloads.

Technical Implementation

• Introduced headObject() definition in ConnectionInterface and implemented it in Rest.
• Updated StorageObject::exists() to use headObject() (HTTP HEAD) instead of getObject() (HTTP GET).
• Updated corresponding snippet and unit test expectations.