refactor(cdn): refactor the billing_option resource and dataSource code style

[jinyangyang222]

What this PR does / why we need it:

commit1: refactor the billing_option resource code style.
commit2: refactor the billing_option data source code style.

Which issue this PR fixes:
(optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close that issue when PR gets merged)
fixes #xxx

Special notes for your reviewer:

Release note:

 refactor the `billing_option` resource and dataSource code style

PR Checklist

• Tests added/passed.

Resource Test

$ make testacc TEST="./huaweicloud/services/acceptance/cdn" TESTARGS="-run TestAccBillingOption_basic"
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test ./huaweicloud/services/acceptance/cdn -v -run TestAccBillingOption_basic -timeout 360m -parallel 4
=== RUN   TestAccBillingOption_basic
=== PAUSE TestAccBillingOption_basic
=== CONT  TestAccBillingOption_basic
--- PASS: TestAccBillingOption_basic (15.40s)
PASS
ok      github.com/huaweicloud/terraform-provider-huaweicloud/huaweicloud/services/acceptance/cdn       15.467s

DataSource Test

$ make testacc TEST="./huaweicloud/services/acceptance/cdn" TESTARGS="-run TestAccDataSourceBillingOption_basic"
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test ./huaweicloud/services/acceptance/cdn -v -run TestAccDataSourceBillingOption_basic -timeout 360m -parallel 4
=== RUN   TestAccDataSourceBillingOption_basic
=== PAUSE TestAccDataSourceBillingOption_basic
=== CONT  TestAccDataSourceBillingOption_basic
--- PASS: TestAccDataSourceBillingOption_basic (8.82s)
PASS
ok      github.com/huaweicloud/terraform-provider-huaweicloud/huaweicloud/services/acceptance/cdn       8.862s

Test case coverage

• Documentation updated.

• Schema updated.

• CheckDeleted.

  • a. During query operation (Read Context)
    aa. Resource not found
    >>>>>> Paste the screenshot here <<<<<<

  • b. During delete/disassociate/unbind operation (Delete Context)
    ba. Resource not found
    >>>>>> Paste the screenshot here <<<<<<

[deer-hang]

/approve
/lgtm

[github-ci-robot]

This PR has been approved by: @deer-hang

/approve
/lgtm