Make block-storage endpoint optional for select commands

Several commands unconditionally initialized the Cinder client even when block-storage was not present in the service catalog. This caused failures such as “public endpoint for block-storage service not found” on clouds without Cinder.

Defer volume client initialization until it is actually needed, using the existing is_volume_endpoint_enabled() helper:

• server create: only load Cinder when –volume, –snapshot, or –block-device-mapping with volume/snapshot sources are used
• extension list: skip block-storage extensions when unavailable
• availability zone list: skip volume availability zones when unavailable

Volume-related options still require Cinder and now fail with a clear error when the endpoint is missing.

Closes-Bug: #2160039