feat: add action for server run commands

[h3adex]

Description

Adds stackit_run_command action to our terraform provider to perform day2 operation on iaas server. Waits for stackitcloud/stackit-sdk-go#10354

Why this is better than null_resource / remote-exec:
In hub-and-spoke and private-network-only topologies we don't have public ips for server to automate day-2 operations like Certificate rotation (see example), Firewall rule reloads or Service restarts. Another important benefit is that run commands are getting logged into our platform audit logs.

Checklist

• Issue was linked above
• Code format was applied: make fmt
• Examples were added / adjusted (see examples/ directory)
• Docs are up-to-date: make generate-docs (will be checked by CI)
• Unit tests got implemented or updated
• Acceptance tests got implemented or updated (see e.g. here)
• Unit tests are passing: make test (will be checked by CI)
• No linter issues: make lint (will be checked by CI)