feat(nodes): add feature-flag-gated --enable-infiniband to sf nodes create

Convert `create` to a `createCreateCommand({ enableInfiniband })` factory
so the call site can thread in the result of the `infiniband-preview`
feature-flag check. When the flag is enabled for the caller's account,
the option is mounted on `sf nodes create` and forwards
`_preview_enable_infiniband: true` to the nodes SDK. When the flag is
disabled, the surface is unchanged.

Author: sigmachirality