feat: servicemonitor in jsonnet deployment

[jouve]

What this PR does

add a servicemonitor to the jsonnet deployment.

it already exists for the chart (example: https://github.com/grafana/mimir/blob/80ee3c17ee134541904fcefc7e9b066b6288348d/operations/helm/charts/mimir-distributed/templates/query-scheduler/query-scheduler-servmon.yaml#L2)

the implementation is very similar to the one found in loki: https://github.com/grafana/loki/blob/main/production/ksonnet/loki/servicemonitor.libsonnet

Which issue(s) this PR fixes or relates to

Fixes #

Checklist

• [ ] Tests updated
• [ ] Documentation added
• [x] CHANGELOG.md updated - the order of entries should be [CHANGE], [FEATURE], [ENHANCEMENT], [BUGFIX]

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[jouve]

I applied @pracucci suggestions and rebased on top of main to remove the conflict in the changelog

[leizor]

The CHANGELOG has just been cut to prepare for the next Mimir release. Please rebase main and eventually move the CHANGELOG entry added / updated in this PR to the top of the CHANGELOG document. Thanks!

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[github-actions[bot]]

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