Feature request: slack_configs.send_resolved a templatable field

What did you do?

I'm trying to set up a receiver that can take a dynamic value for send_resolved, such as:

    ## Route alerts based on slack_channel label
      - name: global_slack_channel_label
        slack_configs:
          - api_url: <some-url>
            channel: '{{ printf \"#%s\" (index ((index .Alerts 0).Labels) \"slack_channel\") }}'
            send_resolved: {{ (index ((index .Alerts 0).Labels)) "send_resolved") | quote | default true | replace "\"" ""  }}

With this, we get a line 894: cannot unmarshal !!map into bool as Alertmanager requires this field to be boolean instead of a tmpl_string. There are various approaches to how this could be done so I'm not worried about this particular template working or not, but the send_resolved field does not allow templating as-is.

What did you expect to see?

I would like to be able to render true/false boolean based on a label in an alert. This would reduce the number of receivers required.

Start at Alertmanager's configuration handling for slack_configs.send_resolved and trace how this field is parsed and validated as a boolean. Review the templating behavior described in the issue and any related configuration tests, then establish behavior that accepts dynamically rendered true or false values while preserving static booleans.