feat(helm): add extraConfigsFiles support for file-based ConfigMap injection

[LeeDongon]

SUMMARY

Adds extraConfigsFiles support to the Superset Helm Chart, enabling users to inject ConfigMap values using file-based configurations.

TESTING INSTRUCTIONS

Place a YAML file (e.g., files/user_rbac.yml) under your Helm chart.

In values.yaml, add:

extraConfigsFiles:
  user_rbac.yml: files/user_rbac.yml
extraConfigMountPath: "/app/configs"

### ADDITIONAL INFORMATION
<!--- Check any relevant boxes with "x" -->
<!--- HINT: Include "Fixes #nnn" if you are fixing an existing issue -->
- [ ] Has associated issue:
- [ ] Required feature flags:
- [ ] Changes UI
- [ ] Includes DB Migration (follow approval process in [SIP-59](https://github.com/apache/superset/issues/13351))
- [x] Migration is atomic, supports rollback & is backwards-compatible
- [ ] Confirm DB migration upgrade and downgrade tested
- [ ] Runtime estimates and downtime expectations provided
- [ ] Introduces new feature or API
- [ ] Removes existing feature or API

[korbit-ai]

I've completed my review and didn't find any issues.

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[github-actions]

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[mistercrunch]

Looks ok to me but would love confirmation from @villebro or @dpgaspar as our maintainers closer to helm

[mistercrunch]

Oh also might need to bump the chart's version number? I tried to look for docs around how to approach changing the helm chart but couldn't find anything. @villebro wondering if you write a short entry for the Contributing portion of the docs so we can refer people to it (?)