Onboard Intake: introduce Intake and 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 (#952) this PR is continuation of the other two components of Intake (ticket) and intake user (ticket).

Checklist

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

[rubenhoenle]

Hi @devanshcache,

thanks for the contribution! To speed up the review process of your changes, would it be possible to split this PR into two parts:

One PR for the stackit beta intake user ... subcommands and one PR for the stackit beta intake instance ... subcommands?

The PR is pretty big in it's current state 😄