Add endpoints to start and stop maintenance in alert receive channel …

[iskhakov]

• Added start_maintenance and stop_maintenance actions to api/internal/v1/alert_receive_channels/ POST endpoint. Use same data as in api/internal/v1/start_maintenance/ and api/internal/v1/stop_maintenance/, except alert_receive_channel_id

curl 'http://localhost:3000/api/plugin-proxy/grafana-oncall-app/api/internal/v1/alert_receive_channels/***/start_maintenance/' \
  -XPOST -H 'Content-Type: application/json' \
  --data-raw '{}' \
  --compressed

curl 'http://localhost:3000/api/plugin-proxy/grafana-oncall-app/api/internal/v1/alert_receive_channels/***/stop_maintenance/' \
  -XPOST -H 'Content-Type: application/json' \
  --data-raw '{}' \
  --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)