Add BackupVaultNotifications resource

[KernelPanicAUS]

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Relates OR Closes #0000

Release note for CHANGELOG:

resource/aws_backup_vault_notifications: Add support for backup event notifications.

Output from acceptance testing:

$  make testacc TEST=./aws TESTARGS='-run=TestAccAwsBackupVaultNotifications_basic'
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test ./aws -v -count 1 -parallel 20 -run=TestAccAwsBackupVaultNotifications_basic -timeout 120m
=== RUN   TestAccAwsBackupVaultNotifications_basic
=== PAUSE TestAccAwsBackupVaultNotifications_basic
=== CONT  TestAccAwsBackupVaultNotifications_basic
--- PASS: TestAccAwsBackupVaultNotifications_basic (26.43s)
PASS
ok      github.com/terraform-providers/terraform-provider-aws/aws       27.998s

[DrFaust92]

duplicate of #12501

[DrFaust92]

duplicate of #12501

[ghost]

I'm going to lock this issue because it has been closed for 30 days ⏳. This helps our maintainers find and focus on the active issues.

If you feel this issue should be reopened, we encourage creating a new issue linking back to this one for added context. Thanks!