[stable/insights-agent] Prometheus plugin: added support to service account annotations

[jdesouza]

Why This PR? a short description of why this PR is needed

Fixes #

Changes Changes proposed in this pull request:

Checklist:

• [x] I have included the name of the chart in the title of this PR in square brackets i.e. [stable/goldilocks].
• [x] I have updated the chart version in Chart.yaml following Semantic Versioning.
• [x] Any new values are backwards compatible and/or have sensible default.
• [x] Any new values have been added to the README for the Chart, or helm-docs --sort-values-order=file has been run for the charts that support it.

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