**What this PR does / why we need it**:
Adds `loki.configStorageType` & `loki.externalConfigSecretName` values
and associated helpers and templates to support choosing either
`ConfigMap` or `Secret` as config medium types.
**Which issue(s) this PR fixes**:
Fixes #8268
**Special notes for your reviewer**:
**Checklist**
- [x] Reviewed the
[`CONTRIBUTING.md`](https://github.com/grafana/loki/blob/main/CONTRIBUTING.md)
guide (**required**)
- [x] Documentation added
- [ ] Tests updated
- [x] `CHANGELOG.md` updated
- [ ] Changes that require user attention or interaction to upgrade are
documented in `docs/sources/upgrading/_index.md`
---------
Signed-off-by: garcia.ryan <[email protected]>
Co-authored-by: Stefan Büringer <[email protected]>
Co-authored-by: Sandeep Sukhani <[email protected]>
