feat(helm): Allow extraObject items as multiline strings

[TheRealNoob]

What this PR does / why we need it:

Allows extraObjects to define multiline string passed through {{ tpl }}, useful for templating field names.

Which issue(s) this PR fixes:
Fixes #12032

Special notes for your reviewer:

Checklist

• Reviewed the CONTRIBUTING.md guide (required)
• Documentation added
• Tests updated
• CHANGELOG.md updated
  • If the change is worth mentioning in the release notes, add add-to-release-notes label
• Changes that require user attention or interaction to upgrade are documented in docs/sources/setup/upgrade/_index.md
• For Helm chart changes bump the Helm chart version in production/helm/loki/Chart.yaml and update production/helm/loki/CHANGELOG.md and production/helm/loki/README.md. Example PR
• If the change is deprecating or removing a configuration option, update the deprecated-config.yaml and deleted-config.yaml files respectively in the tools/deprecated-config-checker directory. Example PR

[CLAassistant]

All committers have signed the CLA.

[TheRealNoob]

I've seen examples of enhancements that bumped the helm chart PATCH version. I bumped the MINOR version because that's what SEMVER dictates, but I'll be happy to change it if you want.

[TheRealNoob]

@JStickler why no more review? I've been waiting on this lately wondering if I shouldn't be updating it? My hesitation is I don't want to do it if it's not going to get attention.

[JStickler]

@TheRealNoob, Michel was getting backed up on other work, removing him as a reviewer just makes it clear to the rest of the team that they should be reviewing this PR. If you could fix the conflicts on this PR, I'll try to get someone from engineering to review for you (I'm the technical writer on the team, so I don't do code reviews, just docs).

[TheRealNoob]

If you could fix the conflicts on this PR, I'll try to get someone from engineering to review for you

Done.

[MichelHollands]

@TheRealNoob Thanks for your contribution. This is failing Helm linting. Can you have a look at this?

[TheRealNoob]

@MichelHollands fixed. I found that the bug was introduced in this commit - fortunately it was easy to fix.

[MichelHollands]

lgtm