[Storage] V4 Signature Support for virtual hosted-style signed

[frankyn]

Tracking issue for work defined in companion document.

New Client Request - Support for virtual hosted-style signed URLs

[tseaver]

Analysis:

• _signing.generate_signed_url_v4 already supports this feature via its api_access_endpoint argument.
• We could add a use_virtual_host flag to Blob.generate_signed_url and Bucket.generate_signed_url which defaults False for now (following Option 2 in the spec). If set to True, then the blob/bucket would pass in the appropriate api_access_endpoint, along with the shorter resource.

[frankyn]

Missing conformance tests