Add CDN load balancer with Cloud Storage buckets as backend

[msampathkumar]

Providing a CDN Load balancer code sample with following features

• Google Storages Buckets for back end
• CDN Object Cache Setting like TTL & other with default setting
• Sample Index & 404 Error pages for CDN LB
• A sample test image for testing lb

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.

[modular-magician]

Oops! It looks like no changelog entry is attached to this PR. Please include a release note block in the PR body, as described in https://github.com/GoogleCloudPlatform/magic-modules/blob/master/.ci/RELEASE_NOTES_GUIDE.md:

```release-note:TYPE
Release note
```

[modular-magician]

Hello! I am a robot who works on Magic Modules PRs.

I have detected that you are a community contributor, so your PR will be assigned to someone with a commit-bit on this repo for initial review.

Thanks for your contribution! A human will be with you soon.

@rileykarson, please review this PR or find an appropriate assignee.

[modular-magician]

Oops! It looks like no changelog entry is attached to this PR. Please include a release note block in the PR body, as described in https://github.com/GoogleCloudPlatform/magic-modules/blob/master/.ci/RELEASE_NOTES_GUIDE.md:

```release-note:TYPE
Release note
```

[modular-magician]

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

Diff report:

Terraform GA: Diff ( 2 files changed, 163 insertions(+), 18 deletions(-))
Terraform Beta: Diff ( 3 files changed, 341 insertions(+), 31 deletions(-))
TF OiCS: Diff ( 4 files changed, 236 insertions(+))
TF Cloud Doc Samples: Diff ( 1 file changed, 137 insertions(+))

[modular-magician]

I have triggered VCR tests in RECORDING mode for the following tests that failed during VCR: TestAccApigeeEnvironmentIamMemberGenerated|TestAccApigeeEnvironmentIamPolicyGenerated|TestAccCloudFunctionsFunction_vpcConnector|TestAccComposerEnvironment_composerV1MasterAuthNetworksUpdate|TestAccComposerEnvironment_composerV2MasterAuthNetworksUpdate|TestAccComputeGlobalForwardingRule_externalCndLbWithBackendBucketExample|TestAccComputeForwardingRule_update|TestAccContainerNodePool_withInvalidUpgradeSettings|TestAccServiceNetworkingPeeredDNSDomain_basic You can view the result here: https://ci-oss.hashicorp.engineering/viewQueued.html?itemId=235474

[msampathkumar]

@rileykarson - As the bot(modular-magician) suggested I have updated the release notes. When possible, please check & suggest! Thank you

[msampathkumar]

Commenting for visibility

cc: @rileykarson !