Add existing alerts to terraform

[icco]

Begins #461

Proposed Changes

• Adds a new terraform module you can include to get alerts

Example:

module "en-alerting" {
  source  = "github.com/google/exposure-notifications-verification-server/terraform/alerting"
  project = "example"

  adminapi-host  = "adminapi.example.org"
  apiserver-host = "apiserver.example.org"
  server-host    = "example.org"

  notification-email = "example+alert@google.com"
}

Release Note

There is now an alerting module you can initialize to get alerts for server health.

[google-oss-robot]

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This pull-request has been approved by: icco

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[icco]

/assign @sethvargo
/cc @yegle

[sethvargo]

/lgtm