added data source for storage bucket object content

[edwardmedia]

Fixes hashicorp/terraform-provider-google#3100

If this PR is for Terraform, I acknowledge that I have:

• Searched through the issue tracker for an open issue that this either resolves or contributes to, commented on it to claim it, and written "fixes {url}" or "part of {url}" in this PR description. If there were no relevant open issues, I opened one and commented that I would like to work on it (not necessary for very small changes).
• Generated Terraform, and ran make test and make lint to ensure it passes unit and linter tests.
• Ensured that all new fields I added that can be set by a user appear in at least one example (for generated resources) or third_party test (for handwritten resources or update tests).
• Ran relevant acceptance tests (If the acceptance tests do not yet pass or you are unable to run them, please let your reviewer know).
• Read the Release Notes Guide before writing my release note below.

Release Note Template for Downstream PRs (will be copied)

`google_storage_bucket_object_content`

[modular-magician]

Hi! I'm the modular magician. Your PR generated some diffs in downstreams - here they are.

Diff report:

Terraform GA: Diff ( 5 files changed, 166 insertions(+))
Terraform Beta: Diff ( 5 files changed, 166 insertions(+))

[modular-magician]

I have triggered VCR tests based on this PR's diffs. See the results here: "https://ci-oss.hashicorp.engineering/viewQueued.html?itemId=158834"

[modular-magician]

Hi! I'm the modular magician. Your PR generated some diffs in downstreams - here they are.

Diff report:

Terraform GA: Diff ( 5 files changed, 166 insertions(+))
Terraform Beta: Diff ( 5 files changed, 166 insertions(+))

[modular-magician]

I have triggered VCR tests based on this PR's diffs. See the results here: "https://ci-oss.hashicorp.engineering/viewQueued.html?itemId=158869"

[modular-magician]

I have triggered VCR tests in RECORDING mode for the following tests that failed during VCR: TestAccDataSourceStorageBucketObjectContent_Basic|TestAccDataSourceSpannerInstance_basic|TestAccApiGatewayApiConfigIamBindingGenerated|TestAccApiGatewayApiConfigIamMemberGenerated|TestAccApiGatewayApiConfigIamPolicyGenerated|TestAccApiGatewayGatewayIamBindingGenerated|TestAccApiGatewayGatewayIamMemberGenerated|TestAccApiGatewayGatewayIamPolicyGenerated|TestAccApiGatewayApiConfig_apigatewayApiConfigBasicExample|TestAccApiGatewayApiConfig_apigatewayApiConfigBasicExampleUpdated|TestAccApiGatewayGateway_apigatewayGatewayBasicExample|TestAccApiGatewayGateway_apigatewayGatewayBasicExampleUpdated|TestAccActiveDirectoryDomainTrust_activeDirectoryDomainTrustBasicExample|TestAccBigqueryDataTransferConfig|TestAccComposerEnvironment_withUpdateOnCreate|TestAccFilestoreInstance_filestoreInstanceBasicExample|TestAccFilestoreInstance_update|TestAccLoggingOrganizationSink_described|TestAccLoggingOrganizationSink_disabled|TestAccOSLoginSSHPublicKey_osLoginSshKeyExpiry You can view the result here: "https://ci-oss.hashicorp.engineering/viewQueued.html?itemId=158870"

[modular-magician]

Hi! I'm the modular magician. Your PR generated some diffs in downstreams - here they are.

Diff report:

Terraform GA: Diff ( 5 files changed, 165 insertions(+))
Terraform Beta: Diff ( 5 files changed, 165 insertions(+))

[modular-magician]

I have triggered VCR tests based on this PR's diffs. See the results here: "https://ci-oss.hashicorp.engineering/viewQueued.html?itemId=162889"

[modular-magician]

I have triggered VCR tests in RECORDING mode for the following tests that failed during VCR: TestAccDataSourceComposerEnvironment_basic|TestAccDataSourceGoogleSQLCaCerts_basic|TestAccDataSourceStorageBucketObjectContent_Basic|TestAccDataSourceSqlDatabaseInstance_basic|TestAccDataSourceSpannerInstance_basic|TestAccActiveDirectoryDomainTrust_activeDirectoryDomainTrustBasicExample|TestAccBigqueryDataTransferConfig|TestAccCloudRunService_cloudRunServiceSqlExample|TestAccCloudSchedulerJob_schedulerJobHttpExample|TestAccComposerEnvironment_withUpdateOnCreate|TestAccComputeBackendService_backendServiceNetworkEndpointExample|TestAccFilestoreInstance_filestoreInstanceBasicExample|TestAccFilestoreInstance_update|TestAccOSLoginSSHPublicKey_osLoginSshKeyExpiry|TestAccSQLDatabase_sqlDatabaseBasicExample|TestAccSqlDatabaseInstance_basicSecondGen|TestAccSqlDatabaseInstance_basicMSSQL|TestAccSqlDatabaseInstance_dontDeleteDefaultUserOnReplica|TestAccSqlDatabaseInstance_settings_basic|TestAccSqlDatabaseInstance_settings_deletionProtection|TestAccSqlDatabaseInstance_replica|TestAccSqlDatabaseInstance_highAvailability|TestAccSqlDatabaseInstance_slave|TestAccSqlDatabaseInstance_diskspecs|TestAccSpannerDatabase_basic|TestAccSqlDatabaseInstance_maintenance|TestAccSqlDatabaseInstance_settings_upgrade|TestAccSqlDatabaseInstance_authNets|TestAccSqlDatabaseInstance_settingsDowngrade|TestAccSqlDatabaseInstance_multipleOperations|TestAccSqlDatabaseInstance_basic_with_user_labels|TestAccSqlDatabase_basic|TestAccSqlDatabaseInstance_PointInTimeRecoveryEnabled|TestAccSqlDatabase_update|TestAccSqlClientCert_mysql|TestAccSqlClientCert_postgres|TestAccSqlDatabaseInstance_withPrivateNetwork|TestAccSqlUser_postgres|TestAccSqlUser_postgresAbandon You can view the result here: "https://ci-oss.hashicorp.engineering/viewQueued.html?itemId=162916"