Allow use of dynamic payloads in alert receive channels preview templ…

[iskhakov]

…ate in private api

• Add payload field to api/internal/v1/alert_receive_channels/***/preview_template/ POST endpoint

curl 'http://localhost:3000/api/plugin-proxy/grafana-oncall-app/api/internal/v1/alert_receive_channels/***/preview_template/' \
  -H 'Content-Type: application/json' \
  --data-raw '{"template_name":"web_title_template","template_body":"{{ payload.foo }}", "payload": {"foo": "bar"}}' \
  --compressed

What this PR does

Which issue(s) this PR fixes

Checklist

• Unit, integration, and e2e (if applicable) tests updated
• Documentation added (or pr:no public docs PR label added if not required)
• CHANGELOG.md updated (or pr:no changelog PR label added if not required)