feat(dds): add a datasource to get the versions of DDS instance

[chengxiangdong]

What this PR does / why we need it:

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

• [x] Tests added/passed.
• [x] Documentation updated.
• [x] Schema updated.

Acceptance Steps Performed

make testacc TEST='./huaweicloud/services/acceptance/dds' TESTARGS='-run TestAccDataSourceDdsDatabaseVersions'
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test ./huaweicloud/services/acceptance/dds -v -run TestAccDataSourceDdsDatabaseVersions -timeout 360m -parallel 4
=== RUN   TestAccDataSourceDdsDatabaseVersions_basic
=== PAUSE TestAccDataSourceDdsDatabaseVersions_basic
=== CONT  TestAccDataSourceDdsDatabaseVersions_basic
--- PASS: TestAccDataSourceDdsDatabaseVersions_basic (9.17s)
PASS
ok      github.com/huaweicloud/terraform-provider-huaweicloud/huaweicloud/services/acceptance/dds       9.206s