add support for Extra Manifest Objects

[ghost]

PR Description

Adds the ability to provide additional adhoc Kubernetes manifests.
One of my use cases was injecting ConfigMaps to be mounted as volumes, this could simplify adding dependencies when deploying Alloy.

For example:

controller:
  volumes:
    extra:
      - name: test-volume
        configMap:
          name: hosts
alloy:
  mounts:
    extra:
      - name: test-volume
        mountPath: /etc/test

extraObjects:
  - apiVersion: v1
    kind: ConfigMap
    metadata:
      name: hosts
    data:
      conf.ini: |-
        [hosts]
        localhost = 127.0.0.1

Which issue(s) this PR fixes

n/a

Notes to the Reviewer

It seemed reasonable to me to add the property at root level.
No changelog
No version updated
[x] tested locally

PR Checklist

• CHANGELOG.md updated
• Documentation added
• Tests updated
• Config converters updated

[CLAassistant]

All committers have signed the CLA.