feat: env var MM_SQLSETTINGS_DATASOURCE set (#54)

## Description

- ~~Adds a field to add extra environment settings to the Mattermost
chart.~~ moved to issue

- Sets `MM_SQLSETTINGS_DATASOURCE` so that when restoring from a backed
up Mattermost DB snapshot it uses the newest set connection string,
instead of the previous one from the database snapshot. For more info,
see this [issue](https://github.com/mattermost/mattermost/issues/25696)
on the mattermost github.

## Related Issue

Fixes #53 

## Type of change

- [X] Bug fix (non-breaking change which fixes an issue)
- [X] New feature (non-breaking change which adds functionality)
- [ ] Other (security config, docs update, etc)

## Checklist before merging

- [ ] Test, docs, adr added or updated as needed
- [ ] [Contributor Guide
Steps](https://github.com/defenseunicorns/uds-package-mattermost/blob/main/CONTRIBUTING.md#developer-workflow)
followed

---------

Co-authored-by: Wayne Starr <Racer159@users.noreply.github.com>
Co-authored-by: Jordan McClintock <jordan@defenseunicorns.com>

values/common-values.yaml

@@ -92,6 +92,11 @@ mattermostApp:
         secretKeyRef:
           key: MM_PLUGINSETTINGS_ENABLEUPLOADS
           name: "mattermost-config"
+    - name: MM_SQLSETTINGS_DATASOURCE
+      valueFrom:
+        secretKeyRef:
+          key: db_connection_string
+          name: "mattermost-postgres"
   securityContext:
     runAsUser: 2000
     runAsGroup: 2000