Add staging sub-action for deploying to Fastly staging environment

[aram356]

Summary

• Adds a new fastly/compute-actions/staging composite action that builds and stages a Compute package to the Fastly staging environment without activating it
• The action creates a draft version via fastly compute update --autoclone, optionally adds a comment, then stages it with fastly service-version stage
• service_id is optional — defaults to the value in fastly.toml, consistent with the existing deploy action
• Updates README with staging documentation: sub-action listing, workflow example, and updated inputs table

Test plan

• Verify staging action works with explicit service_id input
• Verify staging action works without service_id (falls back to fastly.toml)
• Verify optional comment is applied to the staged version
• Verify staging action installs CLI via setup sub-action
• Confirm staged version is not activated in production