feat: native support for plugins

[marshall007]

Description

This adds an initContainer used to sideload mattermost plugins. New plugins can be added by adding additional ADD commands to the included Dockerfile.

Related Issue

Fixes #7

Depends on https://github.com/defenseunicorns/uds-common/pull/161 for renovate changes.

Type of change

• [ ] 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 followed

[marshall007]

This is ready for review. Should we drop the melange configs from the PR or keep them around for posterity?