feat: Add support for creating sandboxes from templates and snapshots

This change allows AgentEngineSandboxComputer to create new sandboxes
using either a specified sandbox template or a sandbox snapshot. The
environment variables VMAAS_SANDBOX_TEMPLATE_NAME and
VMAAS_SANDBOX_SNAPSHOT_NAME are introduced to configure this behavior.

Co-authored-by: Emily Feng <emilyfeng@google.com>
Change-Id: Iebdd980a16966ba765cacbce6d63d0d5b691650a