Create a storage node

Optional workspace for this storage node. When provided, the account must be
enabled for the workspace, the caller must be the account owner or a workspace
admin on a public+storage-enabled account, and the node is auto-attached to
that workspace. When omitted, only the account owner may create the node and
no workspace attachment is created (workspaces can be attached afterwards via
POST /storage-nodes/{id}/workspace).

S3 bucket name, Azure container URL, or local path