[consul-exporter] Support using environment variables in command args

[javajawa]

What this PR does / why we need it

To increase the flexibility for options and arguments in the chart, this change adds a flag invokeViaShell to the values. When enabled the consul_exporter is not directly executed by kubernetes. Instead the same command is passed to /bin/sh, which will replace any environment variables.

This allows users to pull in options from secrets on the cluster; kubernetes's downwardApi systems; or other environment ephemera. The option is disabled by default.

This allows for constructs such as

invokeViaShell: true
consulServer: "${CONSUL_HOST}:8500"
extraEnv:
  - name: CONSUL_HOST
    valueFrom:
      fieldRef:
        fieldPath: status.hostIP

Which issue this PR fixes

• fixes [prometheus-consul-exporter] Allow options to use environment variables #6896

Special notes for your reviewer

This change goes slightly against typical helm conventions of having values for command and args which can be overridden by the user.

As we have a always present argument specified by consulServer, I felt that building up the logic for a more generic solution would overall increase the complexity/confusion for this chart.

Checklist

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