Onboard Intake (3/3): introduce Intake user

[devanshcache]

Description

Intake is composed of three components:

• Intake Runners: dedicated, isolated runtime data ingestion environment
• Intakes: a specific data stream or topic within an Intake Runner
• Intake Users: provides secure access credentials for your applications to connect to your Intake

Intake runner has already been merged reference (https://github.com/stackitcloud/stackit-cli/pull/952) this PR is continuation of the other one of two components of intake user (ticket).

Checklist

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