fix: honor .global.postgresql.auth values

[tumido]

Description of the change

Our dependency - bitnami/postgresql chart has an option to configure auth via global scope variables. We do not consider that option when we infer secret name and key for the Backstage deployment volume mounts.

This PR adds support for:

1. .Values.global.postgresql.auth.existingSecret which overrides .Values.postgresql.auth.existingSecret
2. .Values.global.postgresql.auth.secretKeys.userPasswordKey which overrides .Values.postgresql.auth.secretKeys.userPasswordKey and (either of these) gets applied only if
   .Values.global.postgresql.auth.existingSecret or .Values.postgresql.auth.existingSecret is defined

I'm using () chaining Helm ?. pipeline notation/workaround so I can avoid declaring those variables as '' in values.yaml

Existing or Associated Issue(s)

Additional Information

Checklist

• Chart version bumped in Chart.yaml according to semver.
• Variables are documented in the values.yaml and added to the README.md. The helm-docs utility can be used to generate the necessary content. Use helm-docs --dry-run to preview the content.
• JSON Schema generated.
• List tests pass for Chart using the Chart Testing tool and the ct lint command.

[tumido]

cc @vinzscam @sabre1041 @ChrisJBurns

Can you please take a look and let me know what you think? 🙂

[sabre1041]

LGTM. Great work

Approving but added a comment for your consideration

[sabre1041]

LGTM

[ChrisJBurns]

I get scared when I see postgres auth secret values getting changed 😆, mainly because of past stress of trying to satisfy all of the scenarios that tend to cause regression to other values if not done right. Do we have good tests to ensure that when certain values are set, the correct Deployment yaml is generated? As theres a few conditional branches in that backstage.postgresql.databaseSecretKey function. If not, would be good to get these in another time if possible. Will allow us to change any code in this area with increased confidence in future.

I will add though, changes lgtm 👍

[tumido]

I don't have any tests at this time @ChrisJBurns . I've logged an issue to address this in future. I fully agree. 🙂