Add custom_http_statuses

[yktakaha4]

ref: louy#89

Added test to PR submitted to louy/terraform-provider-uptimerobot.
It is a very good fix and I hope it will be merged.

In my local:

$ TF_ACC=1 go test -v ./... -run TestUptimeRobotDataResourceMonitor_custom_http_statuses
?       github.com/vexxhost/terraform-provider-uptimerobot      [no test files]
=== RUN   TestUptimeRobotDataResourceMonitor_custom_http_statuses
2022/09/06 20:39:55 [DEBUG] POST https://api.uptimerobot.com/v2/newMonitor
2022/09/06 20:39:58 [DEBUG] POST https://api.uptimerobot.com/v2/getMonitors
2022/09/06 20:40:00 [DEBUG] POST https://api.uptimerobot.com/v2/getMonitors
2022/09/06 20:40:04 [DEBUG] POST https://api.uptimerobot.com/v2/getMonitors
2022/09/06 20:40:06 [DEBUG] POST https://api.uptimerobot.com/v2/deleteMonitor
2022/09/06 20:40:07 [DEBUG] POST https://api.uptimerobot.com/v2/getMonitors
--- PASS: TestUptimeRobotDataResourceMonitor_custom_http_statuses (15.61s)
PASS
ok      github.com/vexxhost/terraform-provider-uptimerobot/internal/provider    15.622s
?       github.com/vexxhost/terraform-provider-uptimerobot/internal/provider/api        [no test files]

[yktakaha4]

merged: lapras-inc#3