feat(dws): add new datasource to query cns of the dws cluster

[wuzhuanhong]

What this PR does / why we need it:

Add new datasource to query CNs under the dws cluster.

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:

PR Checklist

• Tests added/passed.

make testacc TEST=./huaweicloud/services/acceptance/dws TESTARGS='-run TestAccDataSourcClusterCns_basic'
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test ./huaweicloud/services/acceptance/dws -v -run TestAccDataSourcClusterCns_basic -timeout 360m -parallel 4
=== RUN   TestAccDataSourcClusterCns_basic
=== PAUSE TestAccDataSourcClusterCns_basic
=== CONT  TestAccDataSourcClusterCns_basic
--- PASS: TestAccDataSourcClusterCns_basic (19.40s)
PASS
ok      github.com/huaweicloud/terraform-provider-huaweicloud/huaweicloud/services/acceptance/dws       19.471s

• 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 <<<<<<

[wuzhuanhong]

make testacc TEST=./huaweicloud/services/acceptance/dws TESTARGS='-run TestAccDataSourcClusterCns_basic'
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test ./huaweicloud/services/acceptance/dws -v -run TestAccDataSourcClusterCns_basic -timeout 360m -parallel 4
=== RUN TestAccDataSourcClusterCns_basic
=== PAUSE TestAccDataSourcClusterCns_basic
=== CONT TestAccDataSourcClusterCns_basic
--- PASS: TestAccDataSourcClusterCns_basic (15.67s)
PASS
ok github.com/huaweicloud/terraform-provider-huaweicloud/huaweicloud/services/acceptance/dws 15.717s

[Lance52259]

/approve
/lgtm

[github-ci-robot]

This PR has been approved by: @Lance52259

/approve
/lgtm