Provide a reusable setup-datumctl

[ecv]

Context

Follow-up from review on datum-cloud/infra#2728 (review).

The current provisioning/setup-datumctl script installs datumctl and authenticates a service account, but it lives in this repo and is invoked as a raw script. Per @scotwells, this should instead be packaged like google-github-actions/setup-gcloud so any consumer can install datumctl and authenticate with a service account to access their projects.

Goal

Publish a reusable, versioned GitHub Action that:

• Installs a pinned (or input-selectable) datumctl version, cross-platform (Linux/macOS, x86_64/arm64).
• Authenticates a Datum service account from a credentials input, storing the session in the datumctl keyring for subsequent datumctl/kubectl calls.
• Exposes inputs for version, auth hostname, and credentials (tolerating both raw-JSON and base64-encoded credentials, matching the fix in datum-cloud/infra#2728).
• Is consumable by external repos, not just datum-cloud/infra.

Notes

• Likely belongs in its own repo (e.g. datum-cloud/setup-datumctl) for independent versioning/tagging.
• Once shipped, migrate this repo's E2E workflow off provisioning/setup-datumctl to the published action.