[prometheus-json-exporter] Support full relabeling spec in additionalMetricsRelabels

[alien2003]

What this PR does / why we need it

The serviceMonitor additionalMetricsRelabels value only accepts a map of
targetLabel: replacement pairs, each rendered as a replace metric
relabeling. That excludes the rest of the Prometheus relabeling spec (regex,
sourceLabels, drop/keep, labelmap, and so on).

This change lets additionalMetricsRelabels also accept a list of full
relabeling objects, rendered verbatim after the two built-in instance/target
relabelings. It mirrors how additionalRelabels already handles relabelings
in the same template. The value type is detected with kindIs "slice", which
is already used elsewhere in the repo (kube-state-metrics,
kube-prometheus-stack).

serviceMonitor:
  targets:
    - name: example
      url: http://example.com/healthz
      # map form (unchanged)
      additionalMetricsRelabels:
        team: backend
    - name: example2
      url: http://example.com/metrics
      # list form (new)
      additionalMetricsRelabels:
        - sourceLabels: [__name__]
          regex: 'go_.*'
          action: drop

Both per-target and serviceMonitor.defaults accept either form, with the
per-target value taking precedence as before. The map form renders identical
output to today, so this is backward compatible and bumped as a minor version.

A servicemonitor unittest suite is added (the chart had none) covering the map
form, the list form, defaults precedence, and empty map/list inputs.

Which issue this PR fixes

• fixes [prometheus-json-exporter] additionalMetricsRelabels: support full relabeling spec (regex, sourceLabels, actions) #6924

Special notes for your reviewer

Verified with the pinned CI tooling:

helm unittest --strict --file 'unittests/**/*.yaml' charts/prometheus-json-exporter
ct lint --charts charts/prometheus-json-exporter

Checklist

• DCO signed
• Chart Version bumped
• Title of the PR starts with chart name (e.g. [prometheus-couchdb-exporter])