Add backend bucket signed URL key (for CDN) support

[modular-magician]

/cc @emilymye

[emilymye]

make testacc TEST=./google TESTARGS='--run="TestAccComputeBackendBucket"'
==> 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="TestAccComputeBackendBucket" -timeout 240m -ldflags="-X=github.com/terraform-providers/terraform-provider-google/version.ProviderVersion=acc"
=== RUN TestAccComputeBackendBucket_backendBucketBasicExample
=== PAUSE TestAccComputeBackendBucket_backendBucketBasicExample
=== RUN TestAccComputeBackendBucketSignedUrlKey_basic
=== PAUSE TestAccComputeBackendBucketSignedUrlKey_basic
=== RUN TestAccComputeBackendBucket_basicModified
=== PAUSE TestAccComputeBackendBucket_basicModified
=== RUN TestAccComputeBackendBucket_withCdnPolicy
=== PAUSE TestAccComputeBackendBucket_withCdnPolicy
=== CONT TestAccComputeBackendBucket_backendBucketBasicExample
=== CONT TestAccComputeBackendBucket_withCdnPolicy
=== CONT TestAccComputeBackendBucket_basicModified
=== CONT TestAccComputeBackendBucketSignedUrlKey_basic
--- PASS: TestAccComputeBackendBucket_withCdnPolicy (18.04s)
--- PASS: TestAccComputeBackendBucket_backendBucketBasicExample (21.90s)
--- PASS: TestAccComputeBackendBucketSignedUrlKey_basic (25.92s)
--- PASS: TestAccComputeBackendBucket_basicModified (30.25s)
PASS
ok github.com/terraform-providers/terraform-provider-google/google 30.607s

[ghost]

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