feat: Add constants for configOverrides file header and footer keys

[Maleware]

Description

Introducing

pub const CONFIG_OVERRIDE_FILE_HEADER_KEY
pub const CONFIG_OVERRIDE_FILE_FOOTER_KEY

To be used in Superset and Airflow to have unified key names.

Will be used in stackabletech/superset-operator#530, once merged and released

Definition of Done Checklist

• Not all of these items are applicable to all PRs, the author should update this template to only leave the boxes in that are relevant
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Author

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1. Changes are OpenShift compatible
2. CRD changes approved
3. Integration tests passed (for non trivial changes)

Reviewer

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1. Code contains useful comments
2. (Integration-)Test cases added
3. Documentation added or updated
4. Changelog updated
5. Cargo.toml only contains references to git tags (not specific commits or branches)

Acceptance

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1. Feature Tracker has been updated
2. Proper release label has been added

[sbernauer]

Thanks! Field names match https://github.com/stackabletech/decisions/issues/21

[sbernauer]

Can you please add a changelog entry?

[sbernauer]

Can you please also rename the PR to something like feat: Add constants for configOverrides file header and footer keys

[Maleware]

Changelog: 612fbb4

Changed name.