Add annotations support to mailroom chart

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What does this MR do?

Add annotations support to mailroom chart.

This allow to add custom annotations on mailroom pod(s).

Required

For anything in this list which will not be completed, please provide a reason in the MR discussion.

• Merge Request Title and Description are up to date, accurate, and descriptive.
• MR targeting the appropriate branch.
• MR has a green pipeline.
• Documentation created/updated.
• Tests added/updated, and test plan for scenarios not covered by automated tests.
• Equivalent MR/issue for omnibus-gitlab opened.

Reviewers checklist

• MR has a green pipeline on https://gitlab.com/gitlab-org/charts/gitlab.
• Consider downstream impact to the Operator, as per evaluating impact from changes to GitLab Chart.

Tests

Helm values:

gitlab:
  mailroom:
    annotations:
      ad.datadoghq.com/mailroom.logs: >-
        [{
          "source": "gitlab",
          "service": "gitlab-mailroom"
        ]}

Annotations are correctly added to pod(s).