alloydb: added initial_user.password_wo to google_alloydb_cluster

[modular-magician]

closes hashicorp/terraform-provider-google#25595

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alloydb: added write-only support for `initial_user.password_wo` to `google_alloydb_cluster`

Test run locally:

─❯ make testacc TEST=./google/services/alloydb TESTARGS='-run=TestAccAlloydbCluster_withPasswordWo'                                                                                       ─╯
sh -c "'/Users/ramon/go/src/github.com/hashicorp/terraform-provider-google/scripts/gofmtcheck.sh'"
==> Checking that code complies with gofmt requirements...
go vet
TF_ACC_REFRESH_AFTER_APPLY=1 TF_ACC=1 TF_SCHEMA_PANIC_ON_ERROR=1 go test ./google/services/alloydb -v -run=TestAccAlloydbCluster_withPasswordWo -timeout 240m -ldflags="-X=github.com/hashicorp/terraform-provider-google/version.ProviderVersion=acc"
=== RUN   TestAccAlloydbCluster_withPasswordWo
=== PAUSE TestAccAlloydbCluster_withPasswordWo
=== CONT  TestAccAlloydbCluster_withPasswordWo
--- PASS: TestAccAlloydbCluster_withPasswordWo (182.04s)
PASS
ok      github.com/hashicorp/terraform-provider-google/google/services/alloydb  182.841s

Derived from GoogleCloudPlatform/magic-modules#16322