feat(helm): enhance extraObjects

[TheRealNoob]

What this PR does

expands functionality of extraObjects to support list and map structures. Copied this feature from kube-prometheus-stack.

Which issue(s) this PR fixes or relates to

didn't open one

Checklist

• Tests updated.
• Documentation added.
• CHANGELOG.md updated - the order of entries should be [CHANGE], [FEATURE], [ENHANCEMENT], [BUGFIX]. If changelog entry is not needed, please add the changelog-not-needed label to the PR.
• about-versioning.md updated with experimental features.

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Note

Low Risk
Helm templating change scoped to extraObjects rendering with added CI coverage; low blast radius but could affect users relying on edge-case formatting/whitespace in rendered manifests.

Overview
Adds support for defining extraObjects as either a list or a map in the mimir-distributed Helm chart; when a map is provided, the template now ignores keys and renders the values as manifests.

Updates documentation/comments and changelog accordingly, and adds an offline CI fixture plus a generated golden-output test to validate the map-based extraObjects rendering.

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[tacole02]

Changelog LGTM