Add support for AlertStatusOnGone in mackerel_monitor connectivity resource

[handlename]

I added mackerel_monitor.connectivity.0.alert_status_on_gone to support AlertStatusOnGone for connectivity monitor.
https://mackerel.io/ja/api-docs/entry/monitors#create:~:text=%E3%83%AB%E3%83%BC%E3%83%AB%E3%81%AE%E3%83%A1%E3%83%A2%E3%80%82-,alertStatusOnGone,-string

Please review the code 🙏

Output from acceptance testing:

$ make testacc TESTS=TestAccMackerelMonitor_Connectivity
TF_ACC=1 go test -v ./mackerel/... -run TestAccMackerelMonitor_Connectivity -timeout 120m
=== RUN   TestAccMackerelMonitor_Connectivity
=== PAUSE TestAccMackerelMonitor_Connectivity
=== CONT  TestAccMackerelMonitor_Connectivity
--- PASS: TestAccMackerelMonitor_Connectivity (6.38s)
PASS
ok  	github.com/mackerelio-labs/terraform-provider-mackerel/mackerel	6.765s

$ make testacc TESTS=TestAccDataSourceMackerelMonitorConnectivity
TF_ACC=1 go test -v ./mackerel/... -run TestAccDataSourceMackerelMonitorConnectivity -timeout 120m
=== RUN   TestAccDataSourceMackerelMonitorConnectivity
=== PAUSE TestAccDataSourceMackerelMonitorConnectivity
=== CONT  TestAccDataSourceMackerelMonitorConnectivity
--- PASS: TestAccDataSourceMackerelMonitorConnectivity (5.23s)
PASS
ok  	github.com/mackerelio-labs/terraform-provider-mackerel/mackerel	5.620s

[Arthur1]

I have confirmed that it works well. Thank you for your contribution!