features: add data_source_google_compute_backend_bucket

[bdronneau]

Hi,

I try to fix my issue with this PR.
I made a backport in #5699

Testing

 make testacc TEST=./google TESTARGS='-run=TestAccDataSourceComputeBackendBucket_basic'
==> Checking source code against gofmt...                                                                                               
==> Checking that code complies with gofmt requirements...                                                                              
TF_ACC=1 TF_SCHEMA_PANIC_ON_ERROR=1 go test ./google -v -run=TestAccDataSourceComputeBackendBucket_basic -timeout 240m -ldflags="-X=github.com/terraform-providers/terraform-provider-google/version.ProviderVersion=acc"
=== RUN   TestAccDataSourceComputeBackendBucket_basic                                                                                   
=== PAUSE TestAccDataSourceComputeBackendBucket_basic                                                                                   
=== CONT  TestAccDataSourceComputeBackendBucket_basic                                                                                   
--- PASS: TestAccDataSourceComputeBackendBucket_basic (23.22s)                                                                          
PASS                                                                                                                                    
ok      github.com/terraform-providers/terraform-provider-google/google 23.261s 

Release note added with upstreaming PR

[rileykarson]

LGTM. Thanks for contributing! I'll upstream to Magic Modules & merge this PR.

[ghost]

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