Configurable health checking return codes

[christophermaier]

While the current implementation maps health-check hook return codes to specific HTTP codes when queried via HTTP, it may be useful for integrating into other systems if the HTTP code can be influenced via GET request parameters.

Extracted from #6023:

Add GET parameter to the health endpoints that enable control over the return code for the different statuses. Prior art: HashiCorp Vault's health endpoint https://www.vaultproject.io/api/system/health.html. This may be the simplest solution and have the best effort-to-payoff ratio.

[stale]

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