feat(cfw): add data source to get a list of CFW service group members

[profoundwu]

What this PR does / why we need it:
add data source to get a list of CFW service group members
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:

1. add data source to get a list of CFW service group members

PR Checklist

• Tests added/passed.
• Documentation updated.
• Schema updated.

Acceptance Steps Performed

make testacc TEST="./huaweicloud/services/acceptance/cfw" TESTARGS="-run TestAccDataSourceCfwServiceGroupMembers_basic"
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test ./huaweicloud/services/acceptance/cfw -v -run TestAccDataSourceCfwServiceGroupMembers_basic -timeout 360m -parallel 4
=== RUN   TestAccDataSourceCfwServiceGroupMembers_basic
=== PAUSE TestAccDataSourceCfwServiceGroupMembers_basic
=== CONT  TestAccDataSourceCfwServiceGroupMembers_basic
--- PASS: TestAccDataSourceCfwServiceGroupMembers_basic (80.36s)
PASS
ok      github.com/huaweicloud/terraform-provider-huaweicloud/huaweicloud/services/acceptance/cfw       80.424s

[houpeng80]

/lgtm

[Jason-Zhang9309]

/approve

[github-ci-robot]

This PR has been approved by: @Jason-Zhang9309

/approve