Add certificate manager certificates datasource

[modular-magician]

This PR adds the Certificate Manager certificates data source.

Tests:

% make testacc TEST=./google/services/certificatemanager TESTARGS='-run=TestAccDataSourceGoogleCertificateManagerCertificates_'           
TF_ACC=1 TF_SCHEMA_PANIC_ON_ERROR=1 go test ./google/services/certificatemanager -v -run=TestAccDataSourceGoogleCertificateManagerCertificates_ -timeout 240m -ldflags="-X=github.com/hashicorp/terraform-provider-google/version.ProviderVersion=acc"
=== RUN   TestAccDataSourceGoogleCertificateManagerCertificates_basic
=== PAUSE TestAccDataSourceGoogleCertificateManagerCertificates_basic
=== RUN   TestAccDataSourceGoogleCertificateManagerCertificates_regionBasic
=== PAUSE TestAccDataSourceGoogleCertificateManagerCertificates_regionBasic
=== RUN   TestAccDataSourceGoogleCertificateManagerCertificates_managedCertificate
=== PAUSE TestAccDataSourceGoogleCertificateManagerCertificates_managedCertificate
=== RUN   TestAccDataSourceGoogleCertificateManagerCertificates_managedCertificateDNSAuthorization
=== PAUSE TestAccDataSourceGoogleCertificateManagerCertificates_managedCertificateDNSAuthorization
=== RUN   TestAccDataSourceGoogleCertificateManagerCertificates_managedCertificateIssuerConfig
=== PAUSE TestAccDataSourceGoogleCertificateManagerCertificates_managedCertificateIssuerConfig
=== CONT  TestAccDataSourceGoogleCertificateManagerCertificates_basic
=== CONT  TestAccDataSourceGoogleCertificateManagerCertificates_managedCertificateDNSAuthorization
=== CONT  TestAccDataSourceGoogleCertificateManagerCertificates_managedCertificateIssuerConfig
=== CONT  TestAccDataSourceGoogleCertificateManagerCertificates_managedCertificate
=== CONT  TestAccDataSourceGoogleCertificateManagerCertificates_regionBasic
--- PASS: TestAccDataSourceGoogleCertificateManagerCertificates_regionBasic (12.54s)
--- PASS: TestAccDataSourceGoogleCertificateManagerCertificates_managedCertificate (33.43s)
--- PASS: TestAccDataSourceGoogleCertificateManagerCertificates_basic (33.59s)
--- PASS: TestAccDataSourceGoogleCertificateManagerCertificates_managedCertificateDNSAuthorization (59.44s)
--- PASS: TestAccDataSourceGoogleCertificateManagerCertificates_managedCertificateIssuerConfig (93.04s)
PASS
ok      github.com/hashicorp/terraform-provider-google/google/services/certificatemanager       94.189s

Release Note Template for Downstream PRs (will be copied)

`google_certificate_manager_certificates`

Derived from GoogleCloudPlatform/magic-modules#11543