cli/cluster: add --use_public_api to nsc create

[annervisser]

Start moving nsc create onto the public compute API.

Adds a hidden --use_public_api flag (default false) with a --no_use_public_api counterpart, so the default can be flipped in a single line later. When enabled, nsc create creates the instance through the public compute ReservationService.ReserveInstance API (waiting for the reservation to be fulfilled and the instance to become ready) instead of the legacy private InstanceService (api.CreateAndWaitCluster).

This reuses the logic that was implemented for the (now-dropped) nsc reservation create command — see #1868.

Clean split for trivial legacy removal

The public path is fully self-contained in create_publicapi.go. Removing the legacy path later means deleting the else/fall-through in NewCreateCmd and promoting the public branch; the shared output helpers stay.

Correctness safeguards

• The reservation deadline never outlives the client-side wait, and a reservation we stop waiting for is cancelled best-effort, so we never leave an untracked instance.
• --use_public_api requires a positive --wait_timeout (the reservation flow is async and must wait for an instance).
• Explicitly-set nsc create flags the public path does not map yet (--unique_tag, --available_secrets, --internal_extra, --experimental*) are rejected rather than silently dropped. Remove entries from publicAPIUnsupportedFlags as mappings are added.

Shared behavior

Output and side-effect files are shared between both paths via the extracted writeInstanceOutputFiles and printInstanceCreated helpers; the JSON/plain output is byte-for-byte identical to legacy nsc create.

Verification

• build + go vet clean; hidden flags absent from --help; mutual-exclusivity, unsupported-flag, and wait_timeout=0 guards all error correctly.
• Live nsc create --use_public_api (plain and -o json) created ready instances with output identical to legacy nsc create; both destroyed.

Amp-Thread-ID: https://ampcode.com/threads/T-019f1880-ac56-7349-b5df-36566f7fc069