feat(dew): add new data source keypairs

[Jason-Zhang9309]

What this PR does / why we need it:

add new data source keypairs

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 new data source keypairs
2. improve the common filter in utils package to make it support pointer type

PR Checklist

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

Acceptance Steps Performed

make testacc TEST='./huaweicloud/services/acceptance/dew' TESTARGS='-run TestAccDataKpsKeypairs_basic' 
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test ./huaweicloud/services/acceptance/dew -v -run TestAccDataKpsKeypairs_basic -timeout 360m -parallel 4
=== RUN   TestAccDataKpsKeypairs_basic
=== PAUSE TestAccDataKpsKeypairs_basic
=== CONT  TestAccDataKpsKeypairs_basic
--- PASS: TestAccDataKpsKeypairs_basic (12.91s)
PASS
ok      github.com/huaweicloud/terraform-provider-huaweicloud/huaweicloud/services/acceptance/dew       13.004s